Diversity in the MSP-1 gene of Plasmodium falciparum. In A, conserved, semiconserved and variable blocks of the MSP-1 gene are represented as open, hatched and filled boxes, respectively. The location and orientation of primers used for typing and sequencing this locus are also shown. Procedures are described elsewhere . In B, the 27 different MSP-1 alleles found in the analysed sample set from Brazil and Vietnam are represented. Alleles were defined as unique combinations of: (a) allelic types in non-repetitive parts of blocks 2, 4 and 6–16, as determined by PCR typing; (b) repeat haplotypes in K1-type and MAD20-type block 2, as determined by DNA sequencing; and (c) nucleotide polymorphisms in blocks 1, 3 and 17, as determined by DNA sequencing. Alleles and repeat haplotypes were numbered according to Ferreira and colleagues . K1-type repeat haplotypes differ in the number and arrangement of SGT and SGP motifs, while MAD20-type repeat haplotypes differ in the number and arrangement of SGG, SVA, SVT and SKG motifs. Codon numbers are given according to Miller and colleagues .