The Comet (single cell gel electrophoresis) assay primarily measures DNA strand breakage in single cells (Singh et al., 1988). Briefly, cells are suspended in low melting point agarose on a microscope slide. The slides are put in lysing buffer to allow the DNA to unwind and then in electrophoresis buffer. During electrophoresis the broken DNA moves towards the anode forming a Comet tail, with the greater the extent of damage, the greater the tail. Assays can be conducted under neutral or alkaline (> pH 13) conditions. Double-strand breaks are measured under neutral conditions and single-strand breaks under alkaline conditions, where abasic sites and other alkali-labile sites or intermediates in base or nucleotide excision repair can also be detected. There are several good reviews concerning the assay (McKelvey-Martin et al., 1993; Fairbairn et al., 1995; Tice, 1995).