Catalyzes a dehydrogenation to introduce C5-6 double bond into lathosterol in cholesterol biosynthesis. a Delta(7)-sterol + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = a Delta(5),Delta(7)-sterol + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + lathosterol + O2 = 7-dehydrocholesterol + 2 Fe(III)-[cytochrome b5] + 2 H2O The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the sterol desaturase family.