E(tc)1 is a dominant mutation that markedly increases the frequency of terminal DNA elongation by gene conversion in terminally deleted chromosomes. E(tc)1 does not significantly influence the frequency of HeT-A and TART-element attachment to the broken chromosome end and does not significantly influence recombination between directly repeated sequences at the end of the truncated chromosome. D.melanogaster lines that carry the E(tc)1 mutation for many generations have telomeres, consisting of HeT-A and TART-elements, that are longer than those found in wild-type lines.