One possible explanation for this lack of correlation could be that there is a time lag between the onset of the anaemia and the increase in the RBC production, as it takes time for the kidneys to detect the low levels of oxygen in the blood, produce erythropoietin, allow this hormone to travel to the bone marrow, exert its effect on RBC progenitors, and actually to increase the release of reticulocytes. This evidence concerns the gene EPO and anemia.