As a member of the cohesin complex, involved in sister chromatid cohesion from the time of DNA replication in S phase to their segregation in mitosis, a function that is essential for proper chromosome segregation, post-replicative DNA repair, and the prevention of inappropriate recombination between repetitive regions. The cohesin complex may also play a role in spindle pole assembly during mitosis (By similarity). In interphase, cohesins may function in the control of gene expression by binding to numerous sites within the genome (By similarity). May control RUNX1 gene expression. Binds to and represses APOB gene promoter (By similarity). May play a role in embryonic gut development, possibly through the regulation of enteric neuron development (By similarity). May promote apoptosis. Component of the cohesin complex, which consists of an SMC1A/B and SMC3 heterodimer core and 2 non-Smc subunits RAD21 and STAG1/SA1, STAG2/SA2 or STAG3/SA3. Interacts (via N-terminus) with SMC1A; the interaction is direct (By similarity). The cohesin complex interacts with NUMA1. The cohesin complex also interacts with CDCA5, PDS5A and PDS5B; this interaction might regulate the ability of the cohesin complex to mediate sister chromatid cohesion. The interaction with PDS5B is direct and is stimulated by STAG1/SA1. The cohesin complex interacts with the cohesin loading complex subunits NIPBL/Scc2 (via HEAT repeats) and MAU2/Scc4. The cohesin complex interacts with DDX11/ChIR1. Directly interacts with WAPL; this interaction is stimulated by STAG1/SA1. Interacts with the ISWI chromatin remodeling complex component SMARCA5; the interaction is direct (By similarity). Interacts with the NuRD complex component CHD4; the interaction is direct (By similarity). Associates with chromatin. Before prophase, scattered along chromosome arms. During prophase and prometaphase, most cohesins dissociate from the arms of condensing chromosome, possibly through PLK1-mediated phosphorylation (By similarity). A small amount of cohesin remains in centromeric regions and is removed from chromosomes only at the onset of anaphase. At anaphase, cleavage by separase/ESPL1 leads to the dissociation of cohesin from chromosomes and chromosome separation (By similarity). The C-terminal part associates with the head of SMC1A, while the N-terminal part binds to the head of SMC3. Cleaved by separase/ESPL1 at the onset of anaphase; this cleavage is required for sister chromatid separation and cytokinesis. Cleaved by caspase-3/CASP3 or caspase-7/CASP7 at the beginning of apoptosis. Phosphorylated; becomes hyperphosphorylated in M phase of cell cycle. The large dissociation of cohesin from chromosome arms during prophase may be partly due to its phosphorylation by PLK1. Belongs to the rad21 family.