4. (5 points) Using no more than four sentences, explain why the vast majority of HNPCC patients have defects in the MSH2 or MLH1 genes rather than in MSH3, MSH6, PML2 or MLH3.
MSH2 and MLH1 are components of all human MSH and MLH heterodimers, respectively. Therefore they are essential for MSH and MLH function, respectively, and thus essential for mismatch repair. In contrast, MSH3, MSH6, PML2 and MLH3 are components only of subsets of MSH or MLH heterodimers. Because the various MSH and MLH heterodimers overlap in their specificities, MSH3, MSH6, PML2 and MLH3 are functionally redundant, and none of them is essential for mismatch repair.
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