Telomeres of Drosophila consist of multiple copies of LINE-like transposable elements. These elements are assigned to two classes, HeT-A and TART. They are attached to terminal deletions at their 3' end, thus compensating for the absence of telomerase in Drosophila cells. The attachment of HeT-A elements to the X-chromosome terminal deletions of the regulatory region of the yellow gene was studied. It was shown that, in the case of degradation of the yellow promoter sequence (chromosome underreplication), the Het-A promoter located at the 3' end of this element can activate transcription of the gene. The minimal size of the 3'-end HeT-A element sequence sufficient for the yellow expression was shown to be 400 bp. Since the yellow mutation is expressed phenotypically and the gene impairment is not lethal, we created a convenient model genetic system based on this effect. Using this system, the frequency of attachments of the HeT-A elements to the chromosome end can be visually recorded. This frequency varied in a wide range (from 0.2 x 10(-4) to 2 x 10(-3)) and was strain-specific.