Non-heme iron oxygenase involved in sterols biosynthesis by catalyzing the removal of the first methyl group at the C-4 position (By similarity). 4,4-dimethyl-9-beta,19-cyclopropylsterols such as 24-methylenecycloartanol are the preferred substrates (By similarity). 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + 6 Fe(II)-[cytochrome b5] + 5 H(+) + 3 O2 = 4alpha-carboxy-4beta-methyl-5alpha-cholest-7-ene-3beta-ol + 6 Fe(III)-[cytochrome b5] + 4 H2O 24-methylidenelophenol + 6 Fe(II)-[cytochrome b5] + 5 H(+) + 3 O2 = 4alpha-carboxy-ergosta-7,24(24(1))-dien-3beta-ol + 6 Fe(III)-[cytochrome b5] + 4 H2O Interacts with ACBP1. Expressed at low levels in leaves, roots, siliques and flowers. In vegetative organs, confined to vascular tissues of leaves and roots (PubMed:31341004). In flowers, detected in petal vascular tissues, anther filaments and styles (PubMed:31341004). In siliques, present in funiculi (PubMed:31341004). During embryogenesis, expressed from the globular stage to the mature stage in the embryo but not in the endosperm (PubMed:31341004). The histidine box domains may contain the active site and/or be involved in metal ion binding. The double mutant smo1-1 smo1-3 shows no obvious phenotype. Requires a membrane-bound cytochrome b5 as an obligatory electron carrier from NAD(P)H to SMO. Belongs to the sterol desaturase family.