Insertion of a neomycin resistance cassette into exon 8 disrupted the Xrcc1 gene and replaced 8bp of coding sequence. Western blot analysis did not detect XRCC1 protein or truncation products in total protein extracts from cell lines made from 8.5 day embryos that were homozygous for both Xrcc1tm1Rpe and Trp53tm1Tyj.