Cytokine-driven pathways to the c-fosproto-oncogene. When activated by Lyn, Tec phosphorylates “Substrate X” and triggers the signaling pathway linked to the c-fosactivation. Tec and Jak2 can trans-phosphorylate each other. The biological significance of this phosphorylation in the context of c-fos activation mechanism is not settled yet. Jak2 is required for the appropriate function of “Substrate X” as well as for the phosphorylation of STATs. The STATs activation may also have some roles in the regulation of the c-fos transcription.