CFU-S day 12 (CFU-S d12) progenitors in IL11Ra −/− mice. Mean ± SD of CFU-S d12 from two independent experiments. In each experiment, 75,000 bone marrow cells from each of 2 IL11Ra+/+ and IL11Ra−/− mice were administered intravenously to 5 or 6 lethally irradiated recipients. The recipients were killed on day 12 and macroscopic colonies on the spleen surface were counted.