2. History of two or more episodes of acute chest syndrome (ACS) in the 2-year period preceding enrollment despite the institution of supportive care measures (i.e. asthma therapy and/or hydroxyurea). Acute Chest Syndrome (ACS) is defined as new pulmonary alveolar consolidation (or infiltrate) involving at least one complete lung segment associated with acute symptoms including one or more of the following: fever ≥ 38.5 Celsius, chest pain, tachypnea per age adjusted normal, intercostal retractions/nasal flaring/use of accessory muscles of respiration, wheezing, rales or cough that is not attributed to asthma or bronchiolitis.