The Segregation Distorter (SD) system in Drosophila melanogaster causes the transmission of the SD chromosome at the expense of the SD+ chromosome. This occurs through a defect in sperm-specific chromatin condensation of the SD+-bearing spermatids of the SD/SD+ male. The Sd gene encodes a truncated form of RanGAP that is missing a nuclear export signal and is therefore trapped in the nucleus; normally RanGAP is found at the periphery of the nuclear membrane and is required for normal Ran-mediated nuclear transport. The presence of active RanGAP in the nucleus interferes with nuclear export and causes distortion. We show that mutations that affect nuclear import and export can enhance distortion in an SD background, thus verifying that the defect in nuclear transport is responsible for the unequal transmission of chromosomes. In addition, we identify several genes involved in chromatin condensation which also cause distortion in an SD background, opening the way to the dissection of the mechanism of segreg...