Consistently, subsequent functional analyses of mammary tumor cell lines derived from spontaneous mammary lesions arising in tg d16HER2 and FL-HER2 mice demonstrated that d16HER2+ cells exhibited significantly higher MFE and in vitro secondary and tertiary mammosphere formation and displayed greater in vivo tumor engraftment in serial dilution conditions than their FL-HER2+ cell counterparts, highlighting their CSC-like traits [36,38] (Figure 3). The gene discussed is ERBB2; the disease is breast cancer.