What is the role of JAK Tyrosine Kinase in the JAK-STAT pathway?

• A. JAK Tyrosine Kinase acts as a ligand for the receptor.
• B. JAK Tyrosine Kinase inhibits receptor dimerization.
• C. JAK Tyrosine Kinase binds directly to STAT5.
• D. JAK Tyrosine Kinase phosphorylates the cytokine receptor, activating the signaling pathway.

Answer: (D)