CYCS and thymus atrophy: 28 that PARP1 is an infrequent cause of apoptosis, we did not find any consistent evidence for a role for apoptosis in thymus atrophy by TCDD, as evidenced by effects of TCDD on apoptotic indices (i.e. DNA laddering, PARP or caspase cleavage and cytochrome c release from mitochondria to cytosol).