Describe the expected result of a nonsense mutation in the gene for Mad1
Nonsense mutation of the gene results usually in nonfunctional protein product. The MAD (mitotic arrest deficiency) family of genes codes for proteins that play a role in the mitotic checkpoint to guard cells against aneuploidy. MAD proteins belong to the larger family of a spindle assembly checkpoint (SAC) proteins. Complete loss of SAC proteins is embryonic lethal in mice. Scientists proved positive correlation between the degree of weakness in SAC with the number of aneuploid cells, which emerge and then develop into tumors in mice. Mutant MAD1 may contribute to two major phenotypes of many human cancers, i.e., immortalization and chromosomal instability.