SUMMARYIn the context of studies on the expression ofMhcCyca‐Zsequences of the common carp, PCR amplifications of exon 4 were performed on cDNA obtained from pooled thymi of 20 carp F1 individuals. Five recombinant clones (Cyca‐TC3, ‐TC13, ‐TC15, ‐TC17 and ‐TC18) were found to be 96% similar to the exon 4 region ofCyca‐ZA1. Each of the five sequences was unique, and differed in a few positions in both the nucleotide and the derived amino acid sequences from any of theCyca‐Zsequences known to date. These data suggest that multiple Z genes per locus are present in the carp, which are transcribed in the thymus. In the course of analysing the amplifiedCyca‐Zsequences, serendipity yielded a clone,Cyca‐TC16, containing a class I‐like sequence substantially different from any other carp class I sequence. The predicted amino acid sequence ofCyca‐TC16 was most similar to the class I genes (Lach‐U) from the coelacanth (42–46% amino acid identity).Cyca‐TC16 contains three conserved β2‐microglobulin contact residues, and the secondary structure was predicted by computer algorithms to be similar to that of the α3 domain ofHLA‐A2. Phylogenetic analysis shows that carp class I sequences reside in four distinct clusters: (i)Cyca‐Z, Cyca‐TC3, ‐TC13, ‐TC15, ‐TC17 and ‐TC18 together withCaau‐Zfrom ginbuna crucian carp, (ii)Cyca‐UwithBree‐U(zebrafish) andSasa‐p30 (Atlantic salmon),
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