Model of action of rapamycin on juxtamembrane or catalytic domain mutations of c-kit. Both c-kit mutants (juxtamembrane [c-kit 560] and catalytic domain [c-kit 816]) activate mTOR and consequently its targets 4E-BP1 and p70S6k. Rapamycin induces inhibition of 4E-BP1 phosphorylation only in c-kit 816 cells. This differential effect could be due to an increase of a putative 4E-BP1 phosphatase or inhibition of an alternative 4E-BP1 kinase induced by c-kit 816 mutant. Alternatively, c-kit 560 mutant may have an opposite effect.