||P.146 bottom paragraph: "4-3 Multiple contiguous V gene segments are present at each immunoglobulin locus: For simplicity [researchers] have discussed the formation of a complete V-region sequence as though there were only a single copy of each gene segment. In fact, there are multiple copies of all the gene segments in germline DNA. It is the random selection of just one gene segment of each type that makes possible the great diversity of V regions among immunoglobulins. The numbers of functional gene segments of each type in the human genome, as determined by gene cloning and sequencing, are shown in Fig. 4.3. Not all the gene segments are functional, as a proportion have accumulated mutations that prevent them from encoding a functional protein. These are termed ‘pseudogenes.’ Because there are many V, D, and J gene segments in germline DNA, no single one is essential. This reduces the evolutionary pressure on each gene segment to remain intact, and has resulted in a relatively large number of pseudogenes. Since some of these can undergo rearrangement just like a normal gene segment, a significant proportion of rearrangements incorporate a pseudogene and will thus be nonfunctional."