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Chromosome 13
Acrocentric - carries BRCA2 and RB1, and is the chromosome involved in Patau syndrome (trisomy 13).
Schematic of chromosome 13 showing the short (p) and long (q) arms. Arm ratio per GRCh38 - banding schematic.
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Overview
Chromosome 13 accounts for roughly 3 to 4 per cent of the total DNA in a human cell and sits within the acrocentric group of chromosomes, sharing structural features with chromosomes 14, 15, 21, and 22. Its acrocentric classification reflects the position of the centromere near one end of the chromosome, creating a very short p arm and a long q arm that houses the vast majority of its protein-coding genes. Clinically, chromosome 13 is recognised for harbouring genes central to tumour suppression and DNA-damage response, particularly BRCA2 and RB1. Pathogenic variants in these genes underpin some of the most extensively researched hereditary cancer syndromes. Beyond single-gene disorders, the presence of an extra copy of chromosome 13 leads to Patau syndrome, a life-limiting condition characterised by severe developmental anomalies. The chromosome also carries ATP7B, pathogenic variants in which cause Wilson disease, a treatable copper-metabolism disorder. Understanding the structure and function of chromosome 13 helps explain both rare chromosomal syndromes and common familial cancer predispositions encountered in NHS clinical genetics services.
Structure
Like all acrocentric chromosomes, chromosome 13 has a centromere positioned very close to one end, dividing it into a diminutive short arm (p arm) and a substantially longer long arm (q arm). The short arm is largely composed of tandemly repeated ribosomal RNA genes that cluster together to form the nucleolar organiser region. During interphase, these ribosomal DNA arrays from multiple acrocentric chromosomes coalesce to form the nucleolus, the cellular factory for ribosome assembly. Because the short arm contributes little unique protein-coding sequence, most medically important genes lie on the q arm, particularly in the 13q12-13q14 region. The centromere itself is built from highly repetitive alpha-satellite DNA and serves as the attachment point for spindle fibres during cell division, ensuring that replicated chromosomes segregate accurately into daughter cells.
At the molecular level, chromosome 13 spans approximately 114 megabases and is tightly packaged through successive layers of organisation. DNA wraps around histone protein octamers to form nucleosomes, which are further compacted into chromatin fibres. During metaphase, when cells prepare to divide, chromatin condenses into the familiar X-shaped chromosome structure visible under a microscope. This extreme condensation allows bulky genetic material to be efficiently distributed, but the underlying sequence remains accessible for gene expression during interphase through dynamic chromatin remodelling.
Video: NIPT statistics explained
Notable regions
The 13q12.13 region houses BRCA2, one of the most clinically significant tumour-suppressor genes in the human genome. BRCA2 orchestrates the repair of double-strand DNA breaks via homologous recombination, a high-fidelity repair pathway essential for genomic integrity. Pathogenic variants in BRCA2 substantially elevate lifetime risks of breast, ovarian, prostate, and pancreatic cancers. Families carrying such variants are typically managed through NHS clinical genetics services, which offer predictive testing, enhanced surveillance, and risk-reducing interventions. BRCA2 sits in a gene-dense segment of the q arm, surrounded by other sequences involved in cell-cycle regulation and DNA metabolism.
Further along the q arm, at 13q14.2, lies RB1, the gene encoding the retinoblastoma protein. RB1 was the first tumour suppressor identified and remains a cornerstone of cancer genetics teaching. The retinoblastoma protein acts as a gatekeeper of the cell cycle, preventing cells from replicating damaged DNA by restraining progression from G1 into S phase. Germline pathogenic variants in RB1 cause hereditary retinoblastoma, a rare childhood eye tumour that can be unilateral or bilateral. Early detection through fundoscopy and imaging is critical, and affected families are managed within specialist paediatric oncology and clinical genetics pathways. Loss of RB1 function is also implicated in other malignancies, including osteosarcoma and small-cell lung cancer.
The short arm of chromosome 13, labelled 13p, is structurally distinct. It consists largely of ribosomal RNA gene repeats shared across all five acrocentric chromosomes. These arrays do not encode unique proteins but are vital for ribosome biogenesis. Variations in the length or copy number of these repeats are generally benign, though certain rearrangements involving the short arms of acrocentric chromosomes can lead to balanced Robertsonian translocations. Such translocations are among the most common structural chromosome abnormalities in humans and may predispose to unbalanced gametes and recurrent pregnancy loss.
Notable genes
BRCA2 stands out as the most widely recognised gene on chromosome 13. It encodes a large nuclear protein that recruits the RAD51 recombinase to sites of DNA double-strand breaks, facilitating error-free repair. Pathogenic variants in BRCA2 are associated with hereditary breast-ovarian cancer syndrome and confer markedly increased risks of several other malignancies. Management typically includes tailored surveillance, risk-reducing mastectomy or salpingo-oophorectomy, and in some cases platinum-based chemotherapy or PARP-inhibitor therapy for established cancers. RB1, another key gene on this chromosome, encodes the retinoblastoma protein, a master regulator of cell proliferation. Germline RB1 variants cause hereditary retinoblastoma, usually presenting in early childhood, and affected individuals require lifelong follow-up due to elevated risks of secondary cancers. ATP7B, located at 13q14.3, encodes a copper-transporting ATPase. Biallelic pathogenic variants in ATP7B lead to Wilson disease, an autosomal recessive disorder characterised by toxic copper accumulation in the liver, brain, and cornea. With early diagnosis and chelation therapy, the prognosis is generally favourable, underscoring the importance of considering Wilson disease in patients with unexplained liver dysfunction or neuropsychiatric symptoms.
Chromosomal disorders
Patau syndrome, caused by trisomy 13, occurs when an individual inherits three copies of chromosome 13 instead of the usual two. This whole-chromosome aneuploidy arises most often from nondisjunction during meiosis, a random error in which chromosome pairs fail to separate correctly. The resulting imbalance in gene dosage disrupts multiple developmental pathways, leading to severe congenital anomalies. Common features include midline facial defects such as cleft lip and palate, holoprosencephaly (incomplete separation of the cerebral hemispheres), and structural heart malformations. Affected infants may also have polydactyly, scalp defects, and profound intellectual disability. Trisomy 13 is considered life-limiting, with many affected pregnancies ending in miscarriage and many liveborn infants having limited survival, often measured in days to months. The NHS Fetal Anomaly Screening Programme includes combined first-trimester screening and non-invasive prenatal testing (NIPT) to assess the risk of trisomy 13, alongside the more common trisomies 18 and 21. A high-risk result prompts the offer of diagnostic testing via chorionic villus sampling or amniocentesis, with karyotyping or chromosomal microarray used to confirm the diagnosis.
Mosaic trisomy 13, in which only a proportion of cells carry the extra chromosome, can present with a milder and more variable phenotype. The clinical features depend on the tissue distribution and percentage of trisomic cells. Partial trisomy 13, resulting from unbalanced translocations or other structural rearrangements, may also occur, and the severity of the condition often correlates with the size and gene content of the duplicated segment. These structural variants are less common than full trisomy but are important to recognise, as they can have implications for recurrence risk in future pregnancies if a parent carries a balanced rearrangement. Genetic counselling through NHS clinical genetics services is essential for families navigating these complex scenarios.
Prenatal screening & diagnosis
Screening for trisomy 13 is integrated into the NHS Fetal Anomaly Screening Programme, which offers combined first-trimester screening to all pregnant women in England. This involves a blood test measuring pregnancy-associated plasma protein A and human chorionic gonadotrophin, combined with ultrasound measurement of nuchal translucency at 11 to 14 weeks of gestation. The results generate a risk estimate for trisomy 13, 18, and 21. Women identified as higher risk are offered non-invasive prenatal testing, a cell-free DNA screening test that analyses foetal DNA fragments circulating in maternal blood. NIPT has very high accuracy, with a detection rate exceeding 99 per cent for trisomy 13, though it remains a screening rather than a diagnostic test. A positive NIPT result for trisomy 13 warrants confirmatory invasive testing. Diagnostic options include chorionic villus sampling, typically performed from 11 weeks, or amniocentesis, usually from 15 weeks onward. Both procedures carry a small risk of miscarriage but provide definitive karyotype or microarray results. Detailed anomaly scanning at around 20 weeks may also detect structural abnormalities associated with trisomy 13, such as holoprosencephaly or cardiac defects, prompting further genetic investigation even in the absence of prior screening.
Inheritance & recurrence
Most cases of trisomy 13 arise de novo, meaning they occur sporadically due to a random error in chromosome segregation during the formation of eggs or sperm. The risk increases with advancing maternal age, reflecting age-related changes in meiotic spindle function and chromosome cohesion. For couples who have had one pregnancy affected by full trisomy 13 due to nondisjunction, the recurrence risk in future pregnancies remains low, approximately 1 per cent higher than the age-related background risk. However, if trisomy 13 results from an unbalanced translocation and one parent carries a balanced rearrangement, the recurrence risk can be substantially higher, sometimes exceeding 10 per cent depending on the specific translocation. Parental karyotyping is therefore recommended following a diagnosis of trisomy 13 to clarify the underlying mechanism and inform future reproductive planning. Families are offered genetic counselling through NHS clinical genetics services to discuss recurrence risks, prenatal diagnosis options, and preimplantation genetic testing where appropriate. For single-gene conditions affecting chromosome 13, such as those caused by pathogenic variants in BRCA2, RB1, or ATP7B, inheritance follows Mendelian patterns. BRCA2 and RB1 variants are inherited in an autosomal dominant manner with variable penetrance, while ATP7B variants causing Wilson disease follow autosomal recessive inheritance.
Frequently asked questions
What is Patau syndrome and how is it detected during pregnancy?
Patau syndrome is caused by the presence of three copies of chromosome 13 instead of two. It leads to severe developmental problems, including brain, heart, and facial abnormalities. The NHS offers combined first-trimester screening and non-invasive prenatal testing (NIPT) to assess the risk of trisomy 13, with diagnostic confirmation available through chorionic villus sampling or amniocentesis if screening suggests increased risk.
If I carry a pathogenic variant in BRCA2, what does that mean for my family?
BRCA2 is inherited in an autosomal dominant pattern, meaning each of your children has a 50 per cent chance of inheriting the variant. Your siblings and parents may also be at risk. NHS clinical genetics services offer predictive testing for at-risk relatives, along with tailored surveillance and risk-reduction strategies for those who carry the variant.
Can chromosome 13 abnormalities be inherited, or do they happen randomly?
Most cases of trisomy 13 occur randomly due to errors in egg or sperm formation and are not inherited. However, if trisomy 13 results from a translocation and a parent carries a balanced rearrangement, there may be a higher recurrence risk. Single-gene conditions on chromosome 13, such as those involving BRCA2 or ATP7B, follow Mendelian inheritance and can run in families.
What is the nucleolar organiser region on chromosome 13?
The nucleolar organiser region is a cluster of ribosomal RNA genes on the short arm of chromosome 13. These genes produce components of ribosomes, the cellular machinery that builds proteins. All five acrocentric chromosomes (13, 14, 15, 21, and 22) carry similar clusters, and together they form the nucleolus inside the cell nucleus.
Are there treatments available for conditions caused by genes on chromosome 13?
Treatment depends on the specific condition. Wilson disease, caused by ATP7B variants, is highly treatable with copper-chelating drugs. Hereditary retinoblastoma due to RB1 variants requires close surveillance and prompt treatment of eye tumours. For BRCA2-related cancer predisposition, options include enhanced screening, risk-reducing surgery, and targeted therapies such as PARP inhibitors. Your NHS clinical genetics team can provide personalised guidance.
Launch-set genes on chromosome 13
Genes on this chromosome that currently have dedicated pages in the library. This list grows as we publish more.