Once the gap genes had been identified at the molecular level it was found that each gap gene is expressed in a band in the early embryo generally correlated with the region that is absent in the mutant. In "Drosophila" the gap genes encode transcription factors, and they directly control the expression of another set of genes involved in segmentation, the pair-rule genes. The gap genes themselves are expressed under the control of maternal effect genes such as "bicoid" and "nanos", and regulate each other to achieve their precise expression patterns.