SMC3 is a member of the cohesin complex and has been found to be mutated in approximately 1% of acute myeloid leukemia and 1% myelodysplastic syndromes. The mutations of SMC3 described tend to be missense mutations that occur throughout the gene. Mutations of the various members of the cohesin complex appear to occur in a mutually exclusive manner. Cases of AML with mutations of the cohesin complex may be associated with mutations of NPM1. Currently there does not appear to be any clear prognosistic impact of cohesin complex gene mutations in AML. Cohesin complex mutations are associated with an unfavorable prognosis in myelodysplastic syndrome, and are more frequently found in patients with high IPSS scores and secondary acute myeloid leukemia.