MRC1 and mismatch repair cancer syndrome 1: The predominant cause of LLS-related MMR deficiency, responsible for up to 80% of LLS cancers, involves double somatic mutations in the same MMR gene, known as biallelic MMR deficiency (Geurts-Giele et al. 2014; Haraldsdottir et al. 2014; Mensenkamp et al. 2014).