STAT5B and choriocarcinoma: Similarly, the co-existence of JAK2 and phosphorylated-JAK2, and STAT5 and phosphorylated STAT5, which are all involved in the mitogenic signaling of EPO, has been found frequently in the cells of malignant tumors of female reproductive organs, stomach, choriocarcinoma, melanoma, and capillary endothelial cells.