The MUC9/OGP gene encodes the oviduct-specific secreted glycoprotein, MUC9/OGP. This glycoprotein is a member of two protein superfamilies: the mucin protein superfamily and the glycosyl hydrolase 18 one. It contains 11 exons and 10 introns. Although MUC9/OGP genes cloned from different mammals display the same structure, species-specific features have been observed for each gene. The mechanisms of transcriptional regulation of the MUC9/OGP gene have been investigated. Comparative studies of the cloned 5’-flanking promoter sequences showed that the homologous MUC9/OGP genes demonstrate conservation of transcription factor-specific cis-elements, but their individual promoters bear species-specific features that determine species-specific expression of a given MUC9/OGP gene. The biosynthetic pathway leading to production of the mature MUC9/OGP glycoprotein has been identified. Overt and covert functional potentials of the MUC9/OGP mucin are discussed.