Lynch syndrome is caused by germline mutations in one of the four DNA mismatch repair genes (MLH1, MSH2, MSH6 and PMS2). Mutations in MSH2 cause concomitant loss of hMSH6, while MLH1 mutations lead to concurrent loss of PMS2. Much less frequent mutations in MSH6 or PMS2 are associated with the isolated loss of the corresponding proteins. We here demonstrate the causative role of the first germline mutation of MSH2, c.1249-1251 dupGTT (p.417 V-418I dupV), associated with normal hMSH2 expression and lack of hMSH6 protein despite a normal MSH6 gene sequence. hMSH6 protein was completely lost only in advanced cancer stages due to two different “second hits,” a whole MSH2 gene deletion and a frame-shifting insertion in the MSH6 (C)8 repeat in the coding sequence.