The core components of the cohesin complex, stromal antigen 2 (STAG2), structural maintenance of chromosomes 1 (SMC1), SMC3, and RAD21, as well as its modulators, PDS5 and NIPBL cohesin loading factor (NIPBL), are collectively mutated in 13% of patients with de novo AML, 21% of patients with secondary AML, and 11% of patients with MDS, where they are associated with poor overall survival (5–10). The gene discussed is SMC3; the disease is acute myeloid leukemia.