FK506 efficiently competes AP1510-mediated cell proliferation, but inefficiently competes FK1012-mediated cell proliferation. MTT assays were performed using clonal populations of Ba/F3 cells expressing F3EpoR236 (left column), F3c-kit432 (middle column), or F1c-kit432 (right column). Cells were grown in the presence of 100 nmol/L FK1012 (upper panels), 100 nmol/L AP1510 (middle panels), or 1% IL-3–containing WEHI conditioned medium (lower panels), plus FK506 at the indicated concentrations. Note that in the presence of WEHI conditioned medium, FK506 failed in all cases to inhibit cell proliferation (lower panels). In contrast, for each clone FK506 exerted a concentration-dependent inhibition of AP1510-mediated cell proliferation (middle panels). Similarly, in a Ba/F3 cell clone expressing F3EpoR236, FK506 produced a concentration-dependent inhibition of FK1012-dependent cell proliferation. In contrast, both Ba/F3 cell clones expressing fusion proteins containing c-kit(F3c-kit432 and F1c-kit432) were resistant to FK506-mediated inhibition of FK1012-dependent cell proliferation. Repeat experiments using the same clones and different clones expressing the same constructs yielded very similar results.