Confirmation of defects in the p53 pathway. (A) Western blot of ATM protein showing reduced ATM expression in ALL87r, similar to the control ataxia telangiectasia cell line. Actin shows equal loading; hRad50, a high–molecular weight protein, demonstrates an absence of sample degradation. (B) Chromatogram of the TP53 gene sequence in ALL 19. The arrow indicates the position of the mutated nucleotide. (i) A heterozygous missense 818G>C mutation was identified in the tumor DNA. (ii) The sequence was wild type at the same position in the germline DNA.