Novel sodium-independent small neutral amino acid transporters which transport L- and D-amino acids. A protein comprising the amino acid sequence represented by SEQ ID NO:1 or 4 or an amino acid derived therefrom by deletion, substitution or addition of one or more amino acids and being capable of sodium-independently transporting L- and D-small neutral amino acids and analogs thereof; a gene encoding the above protein; a method of screening substances inhibiting or promoting the function of the above protein; an antibody against the above protein; and a method of regulating cell function by using the above antibody, function inhibitors, function promoters, etc.