Immunostaining for activated caspase 3, a final effector of apoptosis (Fig. 6a), and BAX, a key pro-apoptotic protein (Fig. 6b), was analyzed by laser scanning cytometry in cultured PPHN and control pulmonary arteries chronically exposed to U46619 in the absence of serum supplementation. The gene discussed is CASP3; the disease is persistent fetal circulation syndrome.