The Drosophila mex1 gene is one of several genes clustered within a 10-kb interval of polytene region 71CD that includes the ecdysone-regulated Eip28/29 gene. mex1 is expressed in several developmental stages, with gene transcripts accumulating initially in 9- to 12-hr embryos. During embryogenesis, mex1 exhibits an endoderm-specific pattern of expression. mex1 transcripts are first detected in the anterior and posterior midgut primordia of stage 12 embryos; subsequently, mex1 mRNA accumulates solely in the differentiating embryonic midgut. DNA sequence analysis reveals that the mex1 gene encodes an unusual cysteine-rich polypeptide.