Identification of a NBN-deficient breast cancer cell line. A. Immunoblot analysis of MRN complex proteins in a wildtype mammary epithelial cell line (MCF10A) and the breast cancer cell lines HCC1395 and HCC1937, each before and after irradiation (6 Gy, 30 min). HCC1395 cells showed some 70% reduction of NBN while MRE11 and RAD50 immunoreactivity remained largely unchanged. Note the mobility shift in NBN after irradiation that can still be observed in HCC1395 cells at the reduced protein level. B. Identification by direct sequencing of the p.R215W (c.643C > T) mutation in genomic DNA from HCC1395 (breast cancer cell line, bottom) and HCC1395 BL (lymphoblastoid cell line, middle) in comparison to wildtype (MCF10A, top). The reverse strand is shown, with the position of the mutation indicated by an arrow. The p.R215W mutation was found in a heterozygous state in HCC1395 BL, and in the apparently homozygous state in HCC1395 (loss of heterozygosity).