Therefore, it was shown that the intracellular substrate-binding site shows a substrate selectivity for receiving small neutral amino acid such as glycine, alanine, serine, threonine and cysteine as same as in the extracellular case. [0146] (10) Confirmation of Human asc-1 [0147] From a plasmid containing cDNA of human asc-1 obtained in Example 1(3), cRNA (RNA complementary to cDNA) was prepared u