the pPRLR isoforms transduce a differentiative signal in response to hGH, pPRL or oPRL. CHO-K1 cells were transiently transfected with βcasLuc, pEF61acZ and either 125 ng hPRLR, 50 ng pPRLR-MP (MP), 50 ng pPRLR-LR2 (LR2) or 50 ng pPRLR-LR4 (LR4). β-Casein induction is expressed as the ratio of luciferase activity with and without lactogen in the presence of insulin and dexamethasone. Luciferase activities were normalized for β-galactosidase activity. Data are presented as mean ± S.E.M. (n = 4). (A) All three pPRLRs showed greater induction in response to hGH and pPRL than oPRL. Combined results from two experiments are presented. Letters a–c: within each receptor, means with different letters are different (P < 0.05). (B) Differential stimulation of the β-casein promoter via the hPRLR in response to lactogens (P < 0.0001). Results are representative of two independent experiments. Letters a–c: mean values for different lactogens are significantly different (P < 0.05).