1q21

1q21 is a cytogenetic band on the long arm of human chromosome 1. The designation refers to a region visible in chromosome spreads after banding and includes subregions such as 1q21.1, 1q21.2, and 1q21.3. The exact genomic coordinates depend on the reference genome assembly used.

The 1q21 region is gene-rich and contains many segmental duplications, which makes it a hotspot for structural rearrangements. Consequently, individuals may carry copy-number variations in 1q21, including microdeletions and microduplications. These rearrangements are often mediated by non-allelic homologous recombination and show variable expressivity and penetrance across individuals and families.

Two recurrent conditions associated with 1q21 rearrangements are 1q21.1 microdeletion syndrome and 1q21.1 microduplication syndrome. Reported features span developmental delay or intellectual disability, learning difficulties, and autism spectrum disorders, as well as a range of congenital anomalies. Phenotypes are highly variable; some carriers may have subtle or no symptoms, while others exhibit distinct craniofacial features, neurodevelopmental issues, or brain-related abnormalities. In some cases, 1q21.1 duplications have been linked to macrocephaly, though findings are variable and overlap with other conditions.

Detection and interpretation of 1q21 rearrangements are typically achieved through chromosomal microarray analysis (array CGH or SNP array) or targeted fluorescence in situ hybridization (FISH). In cytogenetic reports, regions may be described by the band designation (1q21) or by specific genomic coordinates (for example, 1q21.1) to define breakpoints.