The invention provides a transgenic animal having within its genome a transgene construct for gastrointestinal tract specific expression of a protein. In a preferred embodiment, the protein is a phytase or a homologue thereof. Such proteins may be heterologous and may be specifically expressed in the salivary gland of the animal by operably linking the nucleic acid sequence encoding the protein with regulatory sequence including a salivary gland protein promoter/enhancer. Also provided are methods of expressing and producing proteins using such nucleic acid constructs. Further, antibodies specific to such proteins and immunological diagnostic kits are also provided.