Analysis of the molecular basis of antimalarial drug resistance has demonstrated that mutations in the dihydrofolate reductase (dhfr) and dihydropteroate synthase genes are associated with development of SP resistance. It has been assumed that pyrimethamine resistance conferred by multiple mutations arose through stepwise selection of the S108N single mutant (except for the A16V/S108T allele). This single-point mutation decreases the sensitivity of dhfr to pyrimethamine in vitro by≈ 10×(4). Subsequent mutations, such as N51I and C59R, cause additional decreases in the sensitivity of dhfr to pyrimethamine. Parasites with a triplemutant allele (51I/59R/108N) are less sensitive to pyrimethamine in vitro, and patients infected with these parasites have a high probability of not responding to SP treatment (5). Addition of I164L to 51I/59R/108N creates a quadruplemutant allele and decreases the sensitivity of dhfr by≈ 1,000×(4), eliminating the clinical effectiveness of SP, as observed in Southeast Asia and South America. However, the situation in Africa seems to be different because most studies conducted since the mid 1990s have shown the quadruple mutant to be rare, even in areas of intensive pyrimethamine use (6). Increasing SP resistance is principally a result of rapid selection for parasites that carry a triple-mutant allele that arose in Southeast Asia and has spread widely in Africa (7, 8).