Making stripes in the Drosophila embryo.

The striped pattern of expression of the Drosophila primary pair rule genes is controlled by independent regulatory units that give rise to individual stripes. The different stripes seem to respond in a concentration-dependent manner to the different combinations of maternal and gap protein gradients found along the anterior-posterior axis of the early embryo. Thus, the initial periodicity appears to be generated by putting together a series of nonperiodic events.