The homeotic genes of the bithorax and Antennapedia com- plexes (BX-C and ANT-C) encode a family of related tran- scription-regulatory proteins required for the determination of the segmental identities of cells throughout the body

The homeotic genes of the bithorax and Antennapedia complexes (BX-C and ANT-C) encode a family of related transcription-regulatory proteins required for the determination of the segmental identities of cells throughout the body (Scott and Carroll, 1987; Akam, 1987; Ingham, 1988; Scott et al., 1989; Kaufman et al., 1990). Expression of each homeotic gene is activated at the blastoderm stage and restricted to a unique set of cells within stereotypical parasegmental and segmental boundaries along the anteroposterior axis of the embryo (Scott et al., 1983; White and Wilcox, 1984; Beachy et al., 1985; Harding et al., 1985; Kuroiwa et al., 1985; Struhl and Akam, 1985; MartinezArias et al., 1987; Kuziora and McGinnis, 1988a; Pultz et al., 1988; Sánchez-Herrero and Crosby, 1988; Kornfeld et al., 1989; Celniker et al., 1989; Diederich et al., 1989). Development of the normal body plan requires continuous expression of each homeotic gene throughout development. Loss of expression of a homeotic gene at any time during development causes transformation of the segmental identity of cells normally expressing that gene (Lewis, 1963; Morata and García-Bellido, 1976; Struhl, 1981, 1982; Merrill et al., 1987; Diederich et al., 1989). Conversely, ectopic homeotic gene expression can cause transformation of the segmental identities of cells normally not expressing that gene (Schneuwly et al., 1987; González-Reyes and Morata, 1990; González-Reyes et al., 1990; Mann and Hogness, 1990). Establishment and maintenance of stable heritable patterns of homeotic gene expression is therefore essential for development of the normal body plan. Initially, homeotic gene expression patterns are modulated by transiently expressed segmentation gene products (Duncan, 1986; Ingham et al., 1986; Ingham and Martinez-Arias, 1986; White and Lehmann, 1986; Riley et al., 1987; Harding and Levine, 1988; Irish et al. 1989; Ish-Horowicz et al., 1989; Tremml and Bienz, 1989a; Jack and McGinnis, 1990; Reinitz and Levine, 1990). However, expression of the segmentation genes subsides during gastrulation and additional factors are required to sustain homeotic gene expression patterns throughout the rest of development. At least four classes of gene functions are required to maintain the patterns of homeotic gene expression initiated at the blastoderm stage. Sustained repression of each homeotic gene outside its normal expression domain is mediated by cross-regulatory interactions among the 119 Development 117, 119-134 (1993) Printed in Great Britain © The Company of Biologists Limited 1993