18q21

18q21 is a cytogenetic band on the long arm of human chromosome 18, designated as 18q21 in the international system for human cytogenetic nomenclature. It lies within the distal portion of the 18q arm and comprises several subbands, including 18q21.1, 18q21.2, and 18q21.3. The band contains multiple genes, and the exact gene content can vary with subband boundaries and individual chromosomal rearrangements.

Two well-characterized genes located in or near 18q21 are SMAD4 and DCC. SMAD4 encodes a central mediator of the TGF-beta signaling pathway and functions as a tumor suppressor. Pathogenic variants or haploinsufficiency of SMAD4 are associated with juvenile polyposis syndrome and an elevated risk of gastrointestinal cancers. DCC encodes a receptor for netrin-1 involved in axon guidance and has been studied in the context of cancer biology; deletions in the 18q21 region have been examined for potential roles in tumor suppression and tumor progression.

Clinical significance of alterations spanning 18q21 depends on the size and exact boundaries of the rearrangement. Deletions of larger portions of 18q, including this region, are associated with 18q- syndrome, which can include developmental delays, growth retardation, congenital anomalies, and dysmorphic features. Duplications in or near 18q21 can also contribute to variable phenotypes. The region is thus of interest in medical genetics for both developmental disorders and cancer predisposition.

Detection and characterization of alterations in 18q21 are achieved through karyotyping, fluorescence in situ hybridization (FISH), and array-based or sequencing-based copy-number analyses, enabling precise mapping of rearrangements and affected genes.