Cis The term proposed by Haldane (1941), by analogy with chemical isomerism, to describe a double heterozygote in coupling phase, that is, with the two dominant factors derived from one parent and the two recessives from the other. The term is used chiefly for closely linked mutants, to which it was first applied by Pontecorvo (1950). The cis configuration for two recessive mutants, 1 and 2, is +/1+/2, where + stands for the wild-type, and the line separates the contributions from the two parents. In molecular biology refers to an effect on a gene directed by the sequence of that gene in contrast to trans effects which are produced by other factors such as transcription factors encoded by other genes. The terms are commonly used to describe factors that influence gene expression. (Cf. trans.)