Expression of Leydig cell-specific mRNA species in testes of GnRH-null mice following injection of hCG. Levels of mRNA were measured by real-time PCR. Results show expression of each mRNA species following 1, 2 or 4 injections of hCG spaced 12 hours apart. The results for each mRNA species are expressed relative to control (untreated) GnRH-null animals. In each group the mean ± SEM of four animals is shown. Groups with different letter superscripts are significantly (P < 0.05) different.