For 10 years, the regulatory regions of the mouse and rabbit whey acidic protein gene have been used to express heterologous proteins in the milk of transgenic mice, as well as to produce pharmaceutical proteins, on a large scale, in the milk of transgenic livestock. To date, a broad range of expression levels have been detected, and elucidation of the structure-function relationship in these regulatory regions might help to achieve high levels of expression, reproducibly. An extended 5′ regulatory region (17·6 kb v. 6·3 kb) of the rabbit whey acidic promoter resulted in an increased frequency of rabbit whey acidic protein expression in transgenic mice. However, the expression levels were low compared with the high expression levels achieved in both transgenic mice and rabbits using the heterologous κ-casein in the 6·3 kb rabbit whey acidic protein 5′ regulatory region. These results underline the importance of the 3’ downstream regulatory regions, which still need to be better characterized in the whey acidic protein gene.