THE preferential loss of human chromosomes from hybrids between human and mouse somatic cells1 is being used in the assignment of human gene loci to chromosomes2. When the experimental design does not select for the gene product under study, the assignment often assumes that the loss of chromosomes is random. For example, hybrid clones derived from mouse cells and SV40-transformed human skin fibroblasts lose the SV40-induced T antigen only when all but a few human chromosomes have been eliminated3. If the pattern of chromosome loss is random, these results could indicate that SV40 DNA has integrated into the DNA of several chromosomes. They might be attributable, however, to the scheduled elimination of a single pertinent human chromosome relatively late in the evolution of each clone, a possibility supported by the finding of a preferential retention of C-7 in hybrids derived from SV40-transformed human cells and a concordance of this chromosome with the presence of SV40 and T antigen4. Moreover, if the loss of chromosomes were directed, joint segregation of two chromosomes might occur, making it difficult to decide which of the two specified the pertinent gene product being lost simultaneously.
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