THE eukaryotic genome has rececently begun to seem less stable, both in evolution1 and in ontogeny2, than many biologists had assumed. These trends prompted the Institute of Genetics to invite to its annual Spring meeting* speakers on six genetically labile systems: the MHC locus and the immunoglobulin genes of mammals, the genes encoding the surface antigens of trypanosomes, and the genes of plant and animal tumour cells. The MHC locus, which illustrates the evolutionary lability of the vertebrate genome, mil be discussed in detail in a later article. What follows here is concerned with the generation of diversity on a much shorter time-scale, in antibodies during the lifetime of vertebrates, and in the surface antigens of parasitic trypanosomes in the course of an infection.
