Given that hGH and pPRL were more effective than oPRL in activating the β-casein promoter via the three pPRLRs, we investigated whether the binding of these ligands to the pPRLR-MP allele also differed. Competitive displacement analysis revealed that this was the case (F289 = 6.066; P = 0.0034; Fig. 5B), where hGH competed most effectively with 125I-hGH for pPRLR-MP binding sites with a Kd = 5.36 nM (95% CI 3.3–8.7 nM; R2 = 0.95), followed by pPRL with a Kd = 8.4 nM (95% CI 7.2–8.6 nM; R2 = 0.96), while oPRL was least effective with a Kd = 11.0 nM (95% CI 9.7–12.5 nM; R2 = 0.97). These data agree with our finding that oPRL stimulated the least amount of β-casein promoter activity through the pPRLR compared to pPRL or hGH.