Treatment of "E. coli" with psoralen-plus-UV light (PUVA) produces interstrand crosslinks in the cells’ DNA. Cole et al. and Sinden and Cole presented evidence that a homologous recombinational repair process requiring the products of genes "uvrA", "uvrB", and "recA" can remove these crosslinks in "E. coli". This process appears to be quite efficient. Even though one or two unrepaired crosslinks are sufficient to inactivate a cell, a wild-type bacterial cell can repair and therefore recover from 53 to 71 psoralen crosslinks. Eukaryotic yeast cells are also inactivated by one remaining crosslink, but wild type yeast cells can recover from 120 to 200 crosslinks.