Some TfR^-/- chimeras are able to produce IgM⁺ B cells. Splenocytes from control and TfR^-/-RAG2^-/- chimeras are stained with Ly9.1 and IgM. Two of the BM-positive TfR^-/-RAG2^-/- chimeras show IgM⁺ cells, one with a substantial amount, indicating some ability to produce B cells that have completed V(D)J recombination.