CISH and polycythemia: This family and others discovered subsequently were found to have erythrocytosis caused by EPOR-truncating mutations that eliminate portions of the cytoplasmic domain, which later was found to negatively regulate EPOR signaling by recruiting various inhibitory proteins, including the tyrosine phosphatase PTPN6, members of the suppressor of cytokine signaling (SOCS) protein family, SH2B adapter protein 3 (SH2B3, LNK), and the p85 regulatory subunit of PI3K38.