Analysis of single strand break repair in MCF-7 monolayer and mammosphere cell populations. (a) Cells were exposed to 4 Gy 60Co γ-radiation and the relative degree of single-strand breakage (SSB) was determined by alkaline single-cell gel electrophoresis (comet assay) immediately after exposure and at the times indicated after exposure. (b) The 'comets' (n of about 100) were categorized according to the NIH LISTSERV (Comet Assay Interest Group web site) in which type 1 comets display the least DNA damage and type 5 the most. The error bars represent the mean ± standard error of the mean in both panels. The comets of the unirradiated cells are labeled Cont. (c) Expression of proteins involved in SSB repair in response to ionizing radiation. Lysates were prepared from unirradiated cells and from cells harvested one hour after exposure to 1 or 10-Gy 60Co γ-radiation and analyzed by immunoblotting with antibodies against several SSB repair proteins. α-Actin served as a loading control.