CYP27A1 and motor neuron disorder: The expression in brain of the necessary enzymes to interconvert the 25R- and 25S-epimers of 3β,7α-dihydroxycholest-5-en-26-oic acids provides a biosynthetic route to the neuroprotective compound 3β,7α-dihydroxycholest-5-en-(25R)26-oic acid [18,67], providing an explanation for the lack of a motor neuron disease phenotype in the Cyp27a1−/− mouse.