Expression of regulatory genes during secretory differentiation and activation. Dotted lines show genes that decrease at least ten-fold during pregnancy, consistent with adipocyte localization. The solid lines show genes that increase at least two-fold at the onset of lactation with much smaller changes during pregnancy. These genes are likely to be important in initiating metabolic changes at secretory activation. LXR, liver X receptor; P17, day 17 of pregnancy; PPAR, proliferator-activated receptor; PrlR, prolactin receptor; SREBP, sterol regulatory element binding protein.