We have cloned by the polymerase chain reaction a 2.1-kb fragment carrying heavy-chain joining (JH) gene segments and a part of the JH-C mu intron of the rat. Sequencing showed that the rat genome has four functional JH segments and that only a slight divergence has occurred after the separation of rat and mouse. A systematic sequence comparison between the two species and human revealed an additional JH pseudogene in rat and mouse 5' of JH1 which has not been described so far. The implications in evolutionary terms are discussed. In addition, we give an assessment of the misincorporation rate of the Taq polymerase.
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