17q2131

17q21.31 refers to a gene-rich segment on the long arm (q) of chromosome 17, cytogenetic band q21.31. Spanning roughly 0.9 to 1.0 megabase, this region contains several genes of developmental and neurobiological interest, including MAPT, which encodes the tau protein, and KANSL1, whose haploinsufficiency is linked to Koolen-de Vries syndrome. The region is notable for a large inversion polymorphism that creates two common haplotypes, designated H1 and H2. This chromosomal architecture has made 17q21.31 one of the most studied examples of large-scale structural variation in the human genome. The H2 haplotype is particularly common in European populations and is associated with reduced recombination in the region, whereas H1 is more broadly distributed.

Structural variation at 17q21.31 underpins clinically significant copy-number variants. Microdeletions that span part or all of this region, often including KANSL1, cause Koolen-de Vries syndrome, characterized by developmental delay, hypotonia in infancy, characteristic facial features, and varying congenital anomalies; seizures can occur in some individuals. Reciprocal microduplications of the same region give rise to a distinct neurodevelopmental disorder spectrum, with features such as developmental delay and autistic traits that vary in severity among individuals.

The region’s susceptibility to rearrangements is partly due to flanking segmental duplications, which predispose to non-allelic homologous recombination during meiosis. Diagnostic evaluation typically involves chromosomal microarray and, when indicated, targeted testing across the breakpoints. In addition to developmental implications, the presence of MAPT within 17q21.31 links the locus to broader studies of tau biology and neurodegenerative disease, though copy-number–driven phenotypes in this region are primarily developmental rather than late-onset.