Histochemical analysis of the ecdysterone‐regulated expression of the Drosophila genes P1 and LSP‐2

³H‐labeled DNA probes for the ecdysterone‐inducible Drosophila genes P1 and LSP‐2 were hybridized in situ to RNA in sections of embryos and larvae. Intense hybridization was detected specifically in fat body cells of third‐instar larvae and not in other cells of third‐instar larvae nor in any cells at earlier stages. These results confirm the stringent tissue‐ and stage‐specificity of P1 and LSP‐2 expression. Hybridization of both probes occurred to virtually all the cells in the fat bodies, indicating that both genes are expressed in the same cells. Since P1 expression begins several hours later than LSP‐2 expression, and appears to be induced directly by ecdysterone, this finding implies that one or more of the fat body components mediating the response to ecdysterone is gene‐specific.