EPO and anemia (phenotype): The baseline ERFE is expected to exhibita circadian rhythm because it is a downstream responder of endogenousEPO, which follows a circadian rhythm.44 The circadian kinetics of baseline ERFE was described by a time-dependentinput function.24 The higher baseline ERFEin anemic rats could be attributed to increased ERFE production, whichoccurs as a secondary effect of increased endogenous EPO levels inresponse to anemia.