Double-strand-break repair protein rad21 homolog is a protein that in humans is encoded by the RAD21 gene. RAD21 (also known as Mcd1, Scc1, KIAA0078, NXP1, HR21), an essential gene, encodes a DNA double-strand break (DSB) repair protein that is evolutionarily conserved in all eukaryotes from budding yeast to humans.  RAD21 protein is a structural component of the highly conserved cohesin complex consisting of RAD21, SMC1A, SMC3, and SCC3 [ STAG1 (SA1) and STAG2 (SA2) in multicellular organisms] proteins, involved in sister chromatid cohesion.
