Twelve novel single nucleotide polymorphisms (SNPs) were found in the CES2 gene from 153 Japanese individuals, who were administered irinotecan or steroidal drugs. The detected SNPs were as follows:1) SNP, MPJ6_CS2001; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CTGGAACAACTCG/CCTCCCCTCGGAA-3'. 2) SNP, MPJ6_CS2002; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-AACCACCACCGCT/CGATCCTAGCAGG-3'. 3) SNP, MPJ6_CS2003; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-AAATGTTTGTCAA/GGTGGATAAATGA-3'. 4) SNP, MPJ6_CS2004; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCTCCTATCGATC/GCCCCAGCGCGCT-3'. 5) SNP, MPJ6_CS2005; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GCCAGTCCCATCC/TGGACCACACACA-3'. 6) SNP, MPJ6_CS2006; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GCTGGGCAACCCG/AGGCTGAGCGGGG-3'. 7) SNP, MPJ6_CS2007; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CAAGCACGCAACC/TGGCAACTGGGGC-3'. 8) SNP, MPJ6_CS2008; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CATGGAGAGTGGC/TGTGGCCCTCCTG-3'. 9) SNP, MPJ6_CS2009; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCTGTTCTTGGCC/TAGGGCCTTGGGC-3'. 10) SNP, MPJ6_CS2010; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCCATCCCCAGCT/AACAGACTCTCTC-3'. 11) SNP, MPJ6_CS2011; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-TCCACCTGGGGTA/GGATGTTGCCTCC-3'. 12) SNP, MPJ6_CS2013; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GGACTGGGGACCG/AAGGTCTCGGGGG-3'The frequencies were 0.029 for MPJ6_CS2004 and CS2013, 0.026 for MPJ6_CS2009, 0.013 for MPJ6_CS2001, 0.007 for MPJ6_CS2003, and 0.003 for the other 7 SNPs. Among these SNPs, MPJ6_CS2005 (100C>T) resulted in an amino acid alteration (R34W), and MPJ6_CS2007 (579C>T) and MPJ6_CS2008 (765C>T) were synonymous (T193T and G255G, respectively). Furthermore, MPJ6_CS2011 (IVS8-2A>G) resulted in variation at ther 3' splice acceptor site.