ERBB2 and cancer: Recent findings indicate that elevated EGFR and HER2 (ErbB family) levels in cancer cells correlate with an increase in H2O2 levels and global protein sulfenylation.74,75 Moreover, we have previously reported that Cys797 of EGFR undergoes sulfenic acid modification.7 Because of its electrophilic nature, EGFR-Cys797-SOH precludes the covalent bond formation with electrophilic inhibitors like afatinib, resulting in significant loss of overall effectiveness.