Fig. 4. Mammary epithelial expression of MYC from D12.5-D15.5 of pregnancy promotes precocious expression of markers of lactation. (A) Northern hybridization analysis of total RNA extracted from MTB and MTB/TOM mouse mammary glands induced with doxycycline from D12.5-D15.5 of pregnancy and harvested at 1-day increments from D12.5 pregnancy through D1PP. Northern blots were hybridized with probes specific to the human MYC transgene (MYC Tg), endogenous mouse Myc, Csnd and prolactin-inducible protein (Pip). Ethidium bromide staining of 28S RNA is shown as a loading control. (B) Western blot analysis of milk protein expression. Mammary protein lysates from uninduced MTB and MTB/TOM mice at D12.5 pregnancy and MTB and MTB/TOM mice induced from D12.5-D15.5 of pregnancy were separated by SDS-PAGE and transferred to nitrocellulose. Mammary lysates from FVB wild-type virgin mice and mice at D18.5 of pregnancy are included as controls. Membranes were probed with polyclonal antiserum specific to mouse milk proteins and ß-tubulin. Expression of -casein and ß-casein are indicated.