Provided is a genetically modified non-human mammal that comprises an anchor DNA sequence inserted at an endogenous locus of a secretory milk protein gene, wherein the anchor DNA sequence comprises a site-specific recombinase recognition site. Also provided is a genetically modified non-human mammal that comprises a transgene inserted at an endogenous locus of a secretory milk protein gene, wherein the transgene encodes a secretory protein and is operably linked to the endogenous promoter of said secretory milk protein gene, and wherein the transgene is flanked by a pair of site-specific recombinase resulting sites. The genetically modified non-human mammals provided can be used for producing the secreted recombinant protein encoded by the transgene from the milk produced by the genetically modified non-human mammals.