This enzyme is required for electron transfer from NADP to cytochrome P450 in microsomes. It can also provide electron transfer to heme oxygenase and cytochrome B5. NADPH + 2 oxidized [cytochrome P450] = H(+) + NADP(+) + 2 reduced [cytochrome P450] Binds 1 FAD per monomer. Binds 1 FMN per monomer. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the NADPH--cytochrome P450 reductase family. In the N-terminal section; belongs to the flavodoxin family. In the C-terminal section; belongs to the flavoprotein pyridine nucleotide cytochrome reductase family. An alternative upstream Met is found in primates and translation may initiate from the upstream Met which would give rise to a 680-residue protein. However, the upstream codon has a weak Kozak signal while the codon used for translation of the shorter 677-residue sequence has a strong Kozak signal and is widely conserved. In addition, protein sequencing indicates that this is the preferred start codon in vivo. Extended N-terminus.