A model system was studied that was associated with the selective amplification of shortened copies of the mdg3 retrotransposon in cultured cells of Drosophila melanogaster. While full-length mdg3 is present in all species phylogenetically closely related to D. melanogaster, the distribution of its deletion copy mdg3del was shown to be restricted only to D. melanogaster strains. mdg3del appeared to be amplified in two Drosophila cell lines of different origin. A "generalized" (statistically averaged) sequence of the shortened copy from a cell line Schneider2 was determined. The structure of this copy was shown to be the same as in the other tested strains of Drosophila. In addition to the major deletion involving the reverse transcriptase domain and a part of the ribonuclease H domain, several other deletions, insertions, and point substitutions were also revealed in shortened copies. The population of shortened copies was shown to result from the amplification of a single mdg3del copy in the genome of Schneider2 cells. The results obtained suggest that defective copies of the retrotransposon can be trans-mobilized in cultured cells and that the mechanisms of retroamplification in cell cultures and in an organism are different.