The βcasLuc plasmid contains the prolactin responsive rat β-casein promoter (342 bp) subcloned upstream of the luciferase reporter gene in pGL3 (Promega, Madison, WI). The pEF61acZ plasmid (Invitrogen) contains the EF promoter upstream of the lacZ (β-galactosidase) reporter gene, and was used to control for transfection efficiency. The hPRLR-Flag expression construct in pEF6C (Invitrogen) was described previously [27] and expresses the long form of the hPRLR with a C-terminal Flag epitope.