RNA whole-mount in situ hybridization on E12.5 Hoxb9 (A) and Hoxd9 (B) embryos. Arrows point to the dense mesenchyme surrounding the primitive nipples. (C) Results of reverse transcription–PCR analyses with templates from various sources: the mammary gland of a virgin mouse (V), the mammary gland from a mouse at 6.5 days of pregnancy (P6.5), the mammary gland from a mouse at 12.5 days of pregnancy (P12.5), the mammary gland from a mouse that had been lactating for 2 days (L2), the mammary gland from a mouse that had been lactating for 7 days (L7), E12.5 embryos without heads (E), adult liver (Lvr), and a no-template control (N).