A critical role for Stat5 in molecular and clinical progression of prostate cancer (PC) to castrate-resistant and metastatic disease [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25] has been established, and Stat5 has been shown to play a key role in pathogenesis of myeloproliferative disorders [26,27,28,29,30,31,32,33,34,35]. The gene discussed is STAT5B; the disease is pachyonychia congenita.