DNA damage may be induced by both intrinsic and extrinsic factors, the latter involving chemical exposure at workplaces. Upon DNA damage induction, checkpoint kinases such as ATM and ATR, phosphorylate serine 139 of the histone H2AX generating γH2AX, to initiate the damage response pathway. This allows antibodies that act against post-translational modifications, such as γH2AX, to be used for detecting genotoxicity. The aromatic amine 4,4'-methylenebis(2-chloroaniline) (MOCA), which is utilized in industry to produce polyurethane resins, exhibits genotoxicity and induces cancers in animals, including rodents and humans. DNA damage, due to MOCA-induced toxicity during its metabolism, is believed to be first step towards carcinogenesis. However, we failed to detect γH2AX induction by MOCA in cellular systems via western blotting, even when electrophoresis-based methods clearly indicated that physical DNA damage had been induced. In the present study, we utilized RNA sequencing of MOCA-treated cells and controls to elucidate factors underlying the discrepancies observed between these two analytical systems. Our results indicated that H2AX mRNA expression was significantly downregulated; this finding partially clarified the inefficient induction of γH2AX. Although downregulation of γH2AX in the presence of MOCA witnessed in this study was quite unexpected, we believe that this finding prompts researchers to be cautious when screening for genotoxicity using γH2AX only, due to the possibility of genotoxicity being overlooked in some cases. Thus, it appears that a combined approach may be more suitable for detecting genotoxicity.