ESREs maintain their ability to mature despite extensive proliferationex vivo. (A) The capability of ESREs to mature was tested sequentially (open time points) during continued ESRE culture. (B) ESREs from time points listed in panel A were placed into erythroid maturation media, and the kinetics of erythroid cell maturation was analyzed by morphological evaluation of cytospun cells. ESREs maintain the capability to completely mature with the same kinetics after 3 days in erythroid maturation media despite prolonged culture ex vivo.