Cytokine receptors have di erent signaling requirements which ultimately lead to various physiological responses.In an e ort to precisely characterize the molecular determinants involved in the proliferative response mediated by cytokines, we examine dose-dependent proliferation of the bc (GM-CSF, IL-3, IL-5) and homodimeric (G-CSF, TPO) cytokine receptors. Here we report that all cytokine receptors tested activate mostly STAT3 and STAT5. While STAT3 had a positive e ect on bc cytokine receptor dependent proliferation, STAT5 was strongly inhibitory. Similarly, G-CSF and TPO lead to activation of STAT3 and STAT5 but, unlike the bc cytokine receptors, both stimulated cellular growth. On the other hand, Ras activation was necessary for all receptor mediated proliferation with the exception of G-CSF R. Truncated mutants of the receptors intracellular domains were used to delineate the functional domains involved in JAK/ STAT and Ras activation linked to cellular growth. For instance, we revealed a critical role for the speci®c alpha subunit of the bc receptors in triggering receptor activation, STAT3 stimulation and proliferation, while Ras activation originates from the distal intracellular portion of the bc subunit. Finally, we showed that proximal STAT activation is the triggering event of G-CSF and TPO receptor function.