The invention relates to a modified lymphoid cell having gene conversion
fully or partially replaced by hypermutation, wherein said cell has no
deleterious mutations in genes encoding paralogues and analogues of the
RAD51 protein, and wherein said cell is capable of directed and selective
genetic diversification of a target nucleic acid by hypermutation or a
combination of hypermutation and gene conversion. The invention also
relates to a method for diversifying any transgenic target gene in said
cell. Preferably, the target gene is integrated into the immunoglobulin
light or heavy chain locus by targeted integration.