STAG1 is a core component of the cohesin complex that functions in sister chr3 cohesion and chr3 organization 1. The cohesin complex forms a proteinaceous ring structure that maintains chromosome 3 during cell division and regulates gene transcription through chr3 topology control 2. STAG1 works alongside its paralog STAG2, though they have distinct regulatory roles - while both proteins bind shared genomic loci, STAG2 occupies unique binding sites not compensated by STAG1 1. In hematopoietic stem cells, STAG1 and STAG2 display complementary but non-redundant functions in maintaining chr3 accessibility and controlling differentiation programs 2. Loss-of-function mutations in STAG1 cause intellectual developmental disorder, autosomal dominant 47, characterized by syndromic intellectual disability, distinctive facial features including wide mouth and deep-set eyes, and often epilepsy and microcephaly 34. The cohesinopathy phenotype results from disrupted gene regulation rather than solely chr3 segregation defects 3. STAG1 mutations are also implicated in cancer pathogenesis, with cohesin alterations occurring in up to 26% of cancer patients, highlighting its critical role in maintaining genomic stability and proper gene expression 5.