Organization of pCAT4.5. The ORF starts at nucleotide 1,140, with the first in-frame ATG start codon at nucleotide 1,600. The derived amino acid sequence is identical to amino acids 141–976 of human TOP3 protein except for two conserved amino acid changes at positions 434 and 717 of hTOP3. The hTOP3 protein shares 42% amino acid identity with the yeast TOP3 protein. Nucleotides 1–1,139 upstream of the CAT4.5 ORF represent genomic sequence of the human TOP3 gene. Y, active-site tyrosine; UTR, untranslated region.