Double-strand-break repair protein rad21 homolog: 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 RUNX gene expression, including that of RUNX1 and RUNX3 (PubMed:17567667). May play a role in embryonic gut development, possibly through the regulation of enteric neuron development (PubMed:25575569). May promote apoptosis. Component of the cohesin complex, which consists of an SMC1 and SMC3 heterodimer core and 2 non-Smc subunits RAD21 and STAG1/SA1, STAG2/SA2 or STAG3/SA3. Associates with chromatin. Before prophase, scattered along chromosome arms. During prophase and prometaphase, most cohesins dissociate from the arms of condensing chromosome, possibl 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). Detected at the oocyte stage. Expressed throughout the embryo in early embryogenesis, with particularly robust expression in the brain and posterior tail regions at 26 hpf. At 48 hpf, strongly expressed in discrete areas of the brain, the mandibular cartilage and branchial arches, the otic vesicle and developing pectoral fins. Cleaved at the onset of anaphase; this cleavage is required for sister chromatid separation and cytokinesis. Cleaved by caspases at the beginning of apoptosis. May be phosphorylated; may become 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. Morpholino knockdown of the protein causes abnormal embryonic development (PubMed:17567667). At 5 dpf, morphants exhibit delayed food transit along the gut and depletion of enteric neurons (PubMed:25575569). Belongs to the rad21 family.