The Plus-C OBPs were first reported in Drosophila and Anopheles (9, 17) and recently to the pea aphid Acyrthosiphon pisum (Hemiptera) (18) indicating that protein evolution occurred before the divergence of diptera from other species; therefore, these proteins may also be present in other insect orders. The Anopheles gambiae Plus-C OBPs are distinguished from the other classes by their longer sequences and the presence of 12 cysteines in conserved positions, six of which correspond closely to the conserved cysteines residues of the classic OBPs