string: Protein bands that appeared to be differentially expressed between the wild-type and mutant cells were excised from the SDS-PAGE gels and the peptide sequence analysed by liquid chromatography tandem mass spectrometry (Gatlin et al., 1998; Shevchenko et al., 1996) at the Bioanalytical Mass Spectrometry Facility (BMSF), University of New South Wales.