CDKN1B and neoplasm: In contrast, SmoA1 mice with wildtype p27Kip1 lived on average twice as long as mice with a single copy of p27Kip1[5], and mice retaining wildtype p27Kip1 in the heterozygous (Ptc1+/−) background survived significantly longer than counterparts lacking p27Kip1, which succumbed due to an increased tumor incidence [14].