“…The existence of nonautonomous ommatidial inversion phenotypes has been interpreted to suggest that both the JAK/STAT and Wingless pathways, cannot be acting directly, but rather must be exerting their e ects via the production of one or more intermediate molecules of signaling pathways capable of acting at a distance. These factors have been given various names including, the`second signal' (the term we shall use in this review), factor X and Wnt X (Papayannopoulos et al, 1998;Wehrli and Tomlinson, 1998;Zeidler et al, 1999b). The identity of a putative second signal has remained elusive despite the fact that a number of predictions regarding the molecular nature and expression pattern of the second signal can be made (Zeidler et al, 1999b).…”