Part of the TM2L mouse mammary preneoplasia. The TM2L mammary outgrowth is an immortal, p53 wt, weakly tumorigenic, hormone-independent alveolar phenotype. Note the extensive alveolar structures in the absence of hormone stimulation. In contrast, EL11 is an immortal, p53 wt, ductal mouse mammary outgrowth that is hormone-dependent and nontumorigenic. The levels of p21Cip1 and p27Kip1 are 9- and 3-fold lower in TM2L compared to EL11 preneoplasia, respectively. For details, see the article by Said et al. in this issue.