However, PRL is also synthesized and secreted by a broad range of cells, including those of the immune system (DiMattia et al. 1988; Pellegrini et al. 1992), breast cancers (Clevenger et al. 1995), and the lining of the pregnant uterus (Gellersen et al. 1991; Zetser et al. 2001). Here, PRL is linked to breast cancer.