A Model of How Reduced or Absent Expression of Cyclin D3 Can Modulate Erythropoiesis and Cause Increased Erythrocyte Size and Reduced RBC Counts. Model of Sankaran et al. in which reduced numbers of cell divisions due to decreased or absent cyclin D3 during terminal erythropoiesis leads to increased MCV and decreased RBC count. Under normal conditions, terminally differentiating proerythroblasts divide three times with a reduction in erythroblast size at each division before they cease dividing and mature into erythrocytes. When cyclin D3 is decreased or absent, the proerythroblasts divide twice before they cease dividing and mature into erythrocytes. Because the cyclin D3-deficient erythroblasts have divided less, the erythroblasts that mature into erythrocytes are fewer and larger than normal, resulting in fewer but larger erythrocytes.Sankaran VG et al. Genes Dev. 2012;26:2075-2087 © 2012 by Cold Spring Harbor Laboratory Press