EPO receptor (EPO-R), like other members of the haematopoietic receptor family, does not possess an endogenous tyrosine kinase activity. However, its close association with Jak2 tyrosine kinase can induce a rapid tyrosine phosphorylation on a number of proteins [Lacombe 1999]. Hypoxia can induce an increased EPO-R expression, which in turn causes increased sensitivity to EPO [Chin 2000].