Constitutive mismatch repair deficiency (CMMRD, MIM#276300) is a rare autosomal recessive inherited disease caused by pathogenic variants in one of the mismatch repair genes MLH1, MSH2, MSH6 or PMS2 (Bakry et al. 2014; Wimmer et al. 2014, 2017; Suerink et al. 2019; Perez-Valencia et al. 2020; Aronson et al. 2021; Duorno et al. 2021). The gene discussed is MSH6; the disease is autosomal recessive disease.