It has been shown [1,2] that 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 is known as the sodium:solute symporter family (SSF) and currently consists of the following proteins:

• Mammalian Na+/glucose co-transporter.
• Mammalian Na+/myo-inositol co-transporter.
• Mammalian Na+/nucleoside co-transporter.
• Mammalian Na+/neutral amino acid co-transporter.
• Mammalian Na+/iodide co-transporter.
• Mammalian Na+/multivitamin co-transporter.
• Escherichia coli Na+/proline symporter (gene putP).
• Escherichia coli Na+/pantothenate symporter (gene panF).
• Escherichia coli hypothetical protein yidK.
• Escherichia coli hypothetical protein yjcG.
• Bacillus subtilis hypothetical protein ywcA (ipa-31R).

These integral membrane proteins are predicted to comprise at least ten membrane spanning domains. As signature patterns we selected two conserved regions; the first one is located in the fourth transmembrane region and the second one in a loop between two transmembrane regions in the C-terminal part of these proteins.