Pairing-dependent interallelic complementation was first described for the Ultrabithorax gene of the bithorax-complex in Drosophila by Lewis and cited as an example of a new phenomenon that Lewis called the "trans-vection effect." Several different kinds of pairing-dependent gene expression have been observed in Drosophila, and it is now clear that a variety of different molecular mechanisms probably underlie the changes in gene expression that are observed after disrupting chromosome pairing. Transvection in the bithorax-complex appears to result from the ability of cis-regulatory elements to regulate transcription of the promoter on the homologous chromosome. The same phenomenon appears to be responsible for pairing-dependent interallelic complementation at numerous other genes in Drosophila. Some transvection effects are dependent on the presence of wild-type or specific mutant forms of the protein encoded by the zeste trans-regulatory gene, but other transvection effects are zeste-independent. The ease with which chromosome aberrations can disrupt transvection also varies widely among different genes.