The present invention provides proteins having the Na.sup.+/glucose transporter activity, DNAs encoding the proteins, a method of screening for a compound enhancing or inhibiting the activity of the proteins, and compounds obtained by the screening method. The proteins having the same or substantially the same amino acid sequence as the amino acid sequence represented by SEQ ID NO: 1, 15 or 26 are useful as a diagnostic marker for a disease such as diabetes. The compounds enhancing or inhibiting the activity of the proteins, obtained the screening method using the proteins, can be used as a prophylactic and/or therapeutic agent for a disease such as diabetes and hyperlipidemia.