This communication reports the DNA level identification of class I and class II sequences associated with 20 RT1 haplotypes which have been assigned previously to eight RT1 groups. Sixteen to 22 bands in genomic blots hybridized with the mouse pH-2III class I cDNA probe. Only the three RT1khaplotypes associated with identical class I restriction fragment patterns. Differences in restriction bands between putatively identical RT1 haplotypes were either less than or equal to 6%, or greater than 50%, suggesting a relatively high level of recombination between serologically identified RT1.A genes and the majority of class I sequences. Restriction fragment patterns associated with three RT1uhaplotypes differed by less than 6%. However, intra-RT1a,intra-RT1b,and intra-RT1lrestriction fragment differences were between 50 and 64%. In specific cases, different RT1 haplotypes associated with identical class I restriction patterns, e.g., RT1m(MNR) and RT1d(MR); higher resolution confirmed the difference (two bands) between RT1mand RT1d.Results of hybridization with the human DC1 βprobe confirmed that the AVN RT1aand NSD RT1bhaplotypes were generated by recombinations within the vicinity of the RT1.B : RT1.D regions. These results demonstrate that a previous classification of RT1 haplotypes was incomplete and did not include the majority of class I and class II sequences which distinguish RT1 haplotypes.
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