At present the mechanisms of desaturation in animal and plant tissues have not been defined. Desaturases are all associated with membranes with the sole exception of stearyl ACP desaturase which is responsible for the introduction of the first double bond at the 9,10 position of stearic acid. This enzyme will, therefore, hopefully serve as a model enzyme for the purification, characterization of the desaturase and the identification of any cofactors uniquely associated with the desaturation mechanism.