The question of which germ‐line Vϰ genes are expressed was studied by sequencing 70 different cDNA clones from a human spleen library and one clone from a fetal liver library. The sequences were compared to a data base containing all germ‐line Vϰ gene and pseudogene sequences. In addition, 51 rearranged genomic Vϰ genes, 170 cDNA and 74 × proteins from the literature were assigned to specific germ‐line Vϰ genes and included in the comparisons. Not all the known, potentially functional Vϰ genes were found to be expressed, while some genes with minor defects are. The total number of expressed genes is smaller than expected: so far 21 germ‐line genes and 5 pairs of duplicated identical genes are known to be transcribed. The corresponding numbers for rearranged genomic V_H genes and x proteins are 17 plus 4 and 7 plus 7, respectively. A second aim of the study was to find out whether the expressed repertoire contains derivatives of germ‐line Vϰ genes still missing in our data base; no evidence for the existence of such genes was found.

Several cDNA clones contained additional nucleotides between the Vϰ and Jϰ gene segments, which may be germ‐line derived, inserted by terminal deoxynu‐cleotidyl transferase or introduced by other mechanisms. Somatic gene conversion seems not to play a major role in creating the human ϰ gene diversity. Various aspects of the hypermutation of ϰ genes are discussed and the formation of block mutations, i.e. the alterations of two or more adjacent nucleotides is stressed as a remarkable feature of the process.