Erythropoietin (Epo) is a hormone that is produced naturally in the body in response to low O₂ levels in the blood. The intracellular events that occur in response to Epo binding to its cell surface receptor are well characterized. What molecule translocates from the cytosol to the nucleus after (a) JAK2 activates STAT5 and (b) GRB2 binds to the Epo receptor? Why did some endurance athletes use Epo to improve their performance ("blood doping") until it was banned by most sports?