Compared to Krt5-CreER with low-dose tamoxifen treatment, the Krt15-CrePR transgene (1 day after 3 consecutive days of 2 mg RU486 administration) labeled a significantly limited number of basal progenitors within the forestomach (Supplementary Fig. 1b, c), which allowed us to examine the early steps of tumor formation arising from fewer total cells undergoing recombination. Here, KRT15 is linked to neoplasm.