MTAP and familial pancreatic carcinoma: The fact that only 1 of 19 (5.3%) consecutive cancers analyzed for MTAP expression on all tumor-containing whole sections and 1.1% of 181 cases analyzed on our heterogeneity TMA showed heterogeneous MTAP deficiency suggests that homozygous 9p21 deletion either occurs early in the development of affected pancreatic cancers or provides a growth advantage to affected cells that enables a rapid overgrowth of clones lacking this deletion.