Graphical summary of expression of erythropoiesis, and HOX B6 and EPO gene expression from 6.5 to 19.5 days of murine gestation showing how expression is coordinated in all three systems. Note that the decrease in erythropoiesis in the fetal liver is proceeded by a more rapid reduction in HOX B6 and EPO expression. During this time, erythropoiesis is transferred to the fetal spleen and HOX B6 and EPO are transferred to the fetal spleen and fetal kidney.