A cDNA encoding rat 2,3-oxidosqualene: lanosterol cyclase, the enzyme responsible for the complex cyclization/rearrangement step in sterol biosynthesis, was cloned by extensive application of PCRs. Oligonucleotide primers were designed in sense and anti-sense directions corresponding to internal peptide sequences of purified enzyme. Successive PCRs with all possible combinations of these primers yielded a 178-bp fragment, and based on its sequence full nucleotide sequence of cDNA was obtained by a "rapid amplification of cDNA ends" (RACE) method.