Hypothetical relationship revealed by dynamic eQTL mapping. At time (a), genotype has no detectable effect on expression. In response to changing cellular conditions, transcription of gene A is induced, but attenuated by a cis-eQTL (orange asterisk, b). At time (c), the product of gene A regulates a distal locus, leading to phenotypic divergence of the two cells. Importantly, the cis eQTL is no longer detectable and appears only as a trans effect.