Fig. 5. Impaired differentiation of mammary epithelium in Dgat1^–/– mice. (A) Reduced expression of markers of mammary epithelial differentiation in Dgat1^–/– mice. Total mammary gland RNA was extracted from wild-type and Dgat1^–/– virgin mice, and at P18 and L1. Expression of the milk protein genes ß-casein and Wap was analyzed by northern blotting. The expression of keratin 18 (K18) was used as a control for the content of RNA from mammary epithelium. (B) Reduced immunostaining of WAP in Dgat1^–/– mammary glands. Immunostaining of ß-catenin (ß-cat) was used as control. (C) Similar STAT5A expression and phosphorylation were observed in wild-type and Dgat1^–/– mammary glands. Glands from virgin mice and from mice at P18 were dissected, and homogenates were prepared. STAT5A was immunoprecipitated from these samples and analyzed by immunoblotting with antibodies that detect STAT5A or phosphorylated tyrosine residues (PY).