The present invention relates to an expression system comprising a DNA sequence encoding a polypeptide which has a biological activity of human &kgr;-casein, the system comprising a 5′-flanking sequence capable of mediating expression of said DNA sequence. In preferred embodiments the 5′-flanking sequence is from a milk protein gene of a mammal such as a casein gene or whey acidic protein (WAP) gene and the DNA sequence contains at least one intron sequence selected from the intron sequences presented in SEQ ID NO:30. The invention also relates to DNA sequences, replicable expression vectors and cells harboring said vectors, recombinant polypeptide e.g. in glycosylated form, and milk, infant formula or nutrient supplement comprising recombinant polypeptide. The invention further relates to a transgenic non-human mammal such as a mouse, rat, rabbit, goat, sheep, pig, lama, camel or bovine species whose germ cells and somatic cells contain a DNA sequence as defined above as a result of chromosomal incorporation into the non-human mammalian genome, or into the genome of an ancestor of said non-human mammal.