Classical citrullinemia is an autosomal recessive disease caused by a genetic deficiency of argininosuccinate synthetase (ASS). We have previously identified 20 mutations in ASS mRNA of human classical citrullinemia and already established the DNA diagnosis of seven mutations as follows. By Southern blot analysis, each of the alleles with exon 5 or 6 deletion in mRNA appears to involve deletion of genomic DNA from this region. Five mutations involving R304W, G324S, IVS-6(-2) (delta Ex7), IVS-13(+5) (delta Ex13), and delta 13 bp/Ex15&IVS-15 (ins37 b/Ex15&16) are diagnosed by a combination of PCR (or modified PCR) and restriction enzyme digestion. It is important to identify the mutation in genomic DNA for prenatal diagnosis and carrier detection. In the present study, we report a novel missense mutation (R279Q) and a new abnormality in the ASS gene (delta 11 bp/IVS-15). As three missense mutations (R272C, R279Q, and G280R) were found in exon 12, we isolated and sequenced the intron regions surrounding exon 12 to establish a DNA diagnostic test. Although a mutation with a deletion of the first seven bases in exon 16 of mRNA (delta 7 b/Ex16) was found in both Japanese and American patients, the abnormality on the ASS gene was different between the Japanese allele (delta 11 bp/IVS-15) and American allele (IVS-15(-1)). The DNA diagnosis of 47 Japanese alleles with classical citrullinemia showed that the IVS-6(-2) and R304W mutations were found in 49% and 17% of the mutated alleles, respectively. We now have DNA diagnosis systems to detect 14 out of 22 mutations and are performing prenatal diagnosis and carrier detection using genomic DNA on classical citrullinemia.