Evidence of G2/M arrest in MRL cells (B), compared with C57BL/6 (B6) cells (A) is clear and consistent with other regenerating (LG/J; healer congenics) versus nonregenerating (SM/J) mouse models. Besides evidence of a DNA damage response (data not shown), actual DNA damage is obvious, using a comet assay, in MRL cells (D), compared with B6 cells (C).