STAG1

STAG1, short for stromal antigen 1, encodes the SA-1 subunit of the cohesin protein complex in vertebrates. The protein is also referred to as STAG1 or SA-1, and it partners with other cohesin components to form the cohesin ring, alongside SMC1A, SMC3, and RAD21. Cohesin containing STAG1 can be distinguished from cohesin containing STAG2 (SA-2), reflecting distinct subunit compositions with related but sometimes separate functions.

The cohesin complex is essential for sister chromatid cohesion during DNA replication and chromosome segregation in mitosis and meiosis. Beyond its role in cohesion, cohesin also contributes to three-dimensional genome organization, DNA repair, and regulation of gene expression. SA-1–containing cohesin complexes have been reported to display distinct chromatin-binding patterns and regulatory effects compared with SA-2–containing complexes, contributing to cell-type–specific transcriptional programs and developmental processes.

STAG1 localizes to the nucleus and associates with chromatin throughout the cell cycle, with dynamic loading and unloading that accompanies replication and mitosis. The gene is broadly expressed, with higher levels in contexts where robust cohesion and chromatin regulation are needed, such as embryonic development and certain stem cell states. Together with other cohesin subunits, SA-1 participates in loop extrusion and stabilization of chromatin interactions that influence enhancer–promoter communication and gene expression.

Clinical relevance is under ongoing investigation. Alterations in cohesin components, including STAG1, have been detected in various cancers and developmental disorders, though STAG1 mutations are less frequent than those observed in STAG2. Research continues to clarify STAG1’s specific roles in genome stability, development, and disease.