Gene CPD (constitutive photomorphogenesis and dwarf) encodes a cytochrome P450 steroid side-chain hydroxylase (CYP90) involved in biosynthesis of brassinosteroids (BRs) in Arabidopsis thanalia. To dissect the molecular mechanism underlying the biosynthesis and action of the new phytohormone in perennial woody plants, a 1442 bp cDNA and a 1900 bp genomic DNA fragments whose 3'- half was well overlapped with the 5'-half of the former were obtained from a hybrid aspen (Populus tremula x tremudelois) by screening of the cDNA and genomic DNA libraries of this tree using a cDNA fragment from the Arabidopsis CPD gene in the present study. Sequence analysis revealed that the open reading frame comprising the cDNA and genomic DNA fragments encoded a protein of 476 amino acids with a molecular weight of 63 kD. This protein is four amino acid residues longer than Arabidopsis CYP90, and 78.32% identical to the latter. Also it shared high homologies with the Arabidopsis CYP90 in all known functional domains,including steroid-binding region closely related to BR biosynthesis, indicating a high conservatism of the putative CDP gene between annual herb and perennial woody plant species.