Northern blot hybridization shows sex difference in HSD17B4 mRNA levels. A Northern blot of brain and gonad total RNA from male (M) and female (F) zebra finches was hybridized with a 32P-labeled CK313884 probe. The blot shows that the band from the male brain sample is 1.7 times the intensity of that from the female brain sample, suggesting male higher expression of HSD17B4 mRNA. The control GAPDH hybridization indicates that the amount of male and female RNA loaded was roughly equivalent in the brain. The testicular and ovarian samples also show differential expression levels of HSD17B4, but GAPDH is likely expressed at higher levels in the testes than the ovaries, and therefore was not used as a loading control in these tissues .