To ensure that all traced cells originate from Krox-20-positive BC cells, i.e., that Tom expression does not result from de novo activation of Egr2 in other cells populations, we searched for Egr2 expression at E12.5 by in situ hybridization on whole embryos (Figure 1—figure supplement 1A), RT-PCR analysis of total RNA extracted skin (Figure 1—figure supplement 1B), and RNAscope on whole embryos (Figure 1—figure supplement 1C–J). This evidence concerns the gene PRPF6 and breast cancer.