MT1A and infectious otitis media: It is also worth noting that genes coding for detoxification enzymes were altered in both groups: in particular, MT1 (metallothionein 1), CYP2D12 and CYP2D9 (cytochrome P450) were found to be down-regulated in AOM-treated mice, whereas CES1D (carboxylesterase 1D) and CYP2DL2 (cytochrome P450) were up- and down-regulated, respectively, in DSS-treated mice.