Quantitative RT-PCR analysis of specific genes that define human and mouse mammary epithelial subsets. Histograms depicting the relative fold difference in RNA expression between specific mammary epithelial cell subsets relative to other subsets in mouse and human mammary tissue. Expression analysis was relative to 18S rRNA. Examples of genes primarily expressed in the (a) MaSC-enriched subset, (b) luminal progenitor subset, and (c) mature luminal subset. At least three independent samples from either mouse or human mammary cell populations were evaluated for each gene. Data represent mean ± SEM. Statistically significant differences of P < 0.05 (t test) were observed between expression in the basal (MS) versus the two luminal subpopulations (ML and PL) for all genes except mouse Notch4.