Erythrocytosis can be primary, secondary, or mixed. Pathways for each of these types of erythrocytosis are depicted. BFU-1, burst forming unit erythroid; EGLN1, gene encoding prolyl hydroxylase-PHD2; EPAS1, gene encoding HIF-2α; EPO, erythropoietin; EPOR, erythropoietin receptor; HIF, hypoxia-inducible transcription factor; IGF1, insulin-like growth factor-1; IRP1, iron regulatory protein 1; JAK2, Janus kinase 2; STAT 5, signal transducer and activator of transcription; TEMPI, telangiectasias, elevated erythropoietin and erythrocytosis, monoclonal gammopathy, perinephric fluid collections, and intrapulmonary shunting; VHL, von Hippel-Lindau protein.