Tumor growth–promoting Gas6 is delivered by bone marrow–derived cells. For technical reasons, the data shown in panels A through C were generated in different experiments; therefore, WT→WT mice were included as reference controls in each group. Tumor growth of subcutaneous CT26 tumors in (A) WT→WT– and WT→KO bone marrow–transplanted mice (n = 6-8; P = NS) and in (B) WT→WT and KO→KO mice (n = 5; P < .05). The reduced growth of CT26 tumors in KO→KO mice (B) was completely rescued by transplantation of WT bone marrow in Gas6^−/− mice (WT→KO mice; A), to similar levels as observed in WT→WT mice (A). (C) Tumor growth of subcutaneous CT26 tumors in WT→WT and KO→WT mice (n = 5; P < .01). Tumor growth was reduced in KO→WT mice compared with WT→WT mice to a similar extent as in KO→KO mice. Hence, Gas6 expression by bone marrow–derived cells promotes tumor growth. (D) RT-PCR analysis, showing Gas6 expression of CT26 tumors grown in chimeric bone marrow–transplanted mice (n = 5-8). Gas6 mRNA levels were undetectable in KO→KO mice but were rescued in WT→KO mice to nearly similar levels as present in WT→WT mice (#undetectable; n = 7-8; P = NS). Conversely, in KO→WT mice, Gas6 mRNA levels were very low. Thus, bone marrow–derived cells deliver Gas6 into tumors (data were generated by the use of tumors from different experiments shown in panels A-C). (E) RT-PCR analysis showing similar TAM receptor expression in CT26 tumors grown in bone marrow–transplanted mice (n = 6-8; P = NS). BMT indicates bone marrow transplantation.