The head kidney and liver were dissected from six fish per group at the end of 28 days of CdCl2 exposure. Total RNA was extracted from the head kidney and liver using the TRIzol reagent (Invitrogen, USA) according to the manufacturer's instructions. Total RNA concentration and purity was quantified by a spectrophotometer (Thermo Scientific, USA), and the quality was examined by agarose gel electrophoresis.