The isolation of microsomal fractions is a mandatory step. As you know, the induction of cytochrome P450 monooxygenase 1A (CYP1A) is most commonly measured in terms of EROD ( 7-ethoxyresorufin deethylation) activity since this is a very sensitive agent. However, and since ethoxyresorufin-O-deethylase (EROD) can react with other entities (proteins) the measurement using this assay will be incorrect if the microsomal proteins is not purified according to the protocol for EROD.