The mutation sites on &UDelta; 5 desaturase genes in &UDelta; 5 desaturase-defective mutants derived from arachidonic acid-producing Mortierella alpina 1S-4 were identified. The mutations resulted in an amino acid replacement (G189E or W301Stop) and uncorrected transcription caused by recognition of an AG-terminal newly created on C207A gene mutation, resulting in low or no A5 desaturase activity in these mutants.