Integral membrane proteins that mediate the intake of a wide variety of molecules with the concomitant uptake of sodium ions (sodium symporters) can be grouped, on the basis of sequence and functional similarities into a number of distinct families. One of these families currently consists of the following proteins:

• Mammalian sodium/sulfate cotransporter [1].
• Mammalian renal sodium/dicarboxylate cotransporter [2], which transports succinate and citrate.
• Mammalian intestinal sodium/dicarboxylate cotransporter.
• Chlamydomonas reinhardtii putative sulfur deprivation response regulator SAC1 [3].
• Caenorhabditis elegans hypothetical proteins B0285.6, F31F6.6, K08E5.2 and R107.1.
• Escherichia coli hypothetical protein yfbS.
• Haemophilus influenzae hypothetical protein HI0608.
• Synechocystis strain PCC 6803 hypothetical protein sll0640.
• Methanococcus jannaschii hypothetical protein MJ0672.

These transporters are proteins of from 430 to 620 amino acids which are highly hydrophobic and which probably contain about 12 transmembrane regions. As a signature pattern, we selected a conserved region which is located in or near the penultimate transmembrane region.