How does erythropoietin (EPO) function in the JAK-STAT pathway?

• A. EPO directly phosphorylates STAT5.
• B. EPO inhibits JAK Tyrosine Kinase activity.
• C. EPO acts as a transcription factor in the nucleus.
• D. EPO binds to the cytokine receptor, initiating the signaling pathway that leads to hematopoiesis.

Answer: (D)