TfR^-/- cells do not contribute to hematopoietic tissues on a wild-type C57BL/6J background. (A) Representative Southern blots of tissues from 2 E12 chimeras show contribution of TfR^-/- cells (targeted allele) to all tissues except spleen, thymus, and bone marrow. I indicates intestine; Br, brain; Sk, skin; H, heart; K, kidney; Lv, liver; L, lung; P, pancreas; SC, spinal cord; SP, spleen; Te, testis; Th, thymus; M, skeletal muscle; BM, bone marrow; m, marker lane. (B) TfR^+/- and TfR^-/- cell contribution to spleen (S), thymus (T), and bone marrow (BM) in chimeras was re-examined to confirm the absence of TfR^-/- cells in these tissues.