1. Pharmacokinetic (PK) studies: The purpose of these studies is to analyze the body's absorption and clearance of hydroxyurea. Patients participating in these studies will receive a one-time dose of hydroxyurea, with blood sample collection at the following time points: right before the dose of hydroxyurea, 15 minutes, 30 minutes, 60 minutes, 2 hours, and 4 hours after the dose. There will be a total of six blood samples drawn, each about half a teaspoon (2 milliliters).