To further define whether the effects of IL-9 were intrinsic to the macrophage population or a result of exogenous factors linked to differences in tumor growth between the strains, we generated mixed bone marrow chimeras where we injected equal numbers of WT (CD45.1) and Il9r−/− (CD45.2) bone marrow cells into lethally irradiated wild-type recipients (CD45.1+ CD45.2+) (Fig. 4f). This evidence concerns the gene IL9 and neoplasm.