The nucleotide sequence of mouse cDNA for spermidine synthase appeared to contain 75 nucleotides of 5' untranslated region, an open reading frame of 909 nucleotides and 297 nucleotides of 3' untranslated region. The open reading frame encoded a polypeptide of 302 amino acids, displaying 95% similarity to human and 33% similarity to E. coli spermidine synthase. The 3' flanking region contained an unusual polyadenylation signal AATACA.