"Bystander effect" mediated by gap junctions. Human fibroblasts were exposed to a-particle radiation (0.3 cGy, panels B and C) such that only 7% of the cells on average would be traversed by a particle track. Activation of p53 by cellular damage was monitored by following the expression of p21Waf1 by immunofluorescence. Compared to non-irradiated cells (panel A), irradiation gave more than the expected 7% p21-positive cells (panel B). Most of the p21Waf1 expression was eliminated when the cells were treated with lindane (panel C), an inhibitor of gap junctions, indicating cell-cell communication in this radiation response. Azzam et al., PNAS, Jan. 16, 2001, 98(2)