GSTM1 and neoplasm: The likelihood of developing OSCC associated with a decrease in mtDNA copy numbers. Among tobacco–betel quid chewers, a significant association between OSCC risk and mtDNA copy number. Between the mtDNA content variation in cases and controls, a significant difference was observed between GSTT1 null genotypes, GSTM1, and HPV infection. A positive correlation was found between decreased mtDNA content and increased tumor stages.