Because the number of alveolar epithelial cells that are immunoreactive for cCK-18 (Figure 3) or active caspase 3 [8] are far greater than the rare TUNEL positive alveolar epithelial cell found in IPF lungs (data not shown), this is the most likely scenario in IPF lung. Here, CASP3 is linked to idiopathic pulmonary fibrosis.