Structural maintenance of chromosomes (SMC) proteins are part of the cohesin and condensin multisubunit complexes that play a critical role in chromosome organization, segregation, gene regulation, and DNA repair. The core cohesin complex includes a heterodimer of SMC1 and SMC3 that functions by forming a ring-like structure that holds sister chromatids together. Cohesin is important to sister chromatid separation and segregation during mitosis and meiosis and also plays a role in double-strand break repair by homologous recombination.