Insertional mutations in mammals and mammalian cells.

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.