Vesicles derived from the endoplasmic reticulum, which contain cytochrome P450 enzymes that are responsible for many phase I biotransformation reactions. Microsomes are isolated by ultracentrifugation (104,000 g for one hour) of the postmitochondrial (S9) fraction.

Structures derived from the endoplasmic reticulum of cell nuclei and used to determine the effect of candidate drug substances in cells.