Milk protein gene expression is multihormonally regulated. Insulin and hydrocortisone are required for the transcription of these genes, whereas prolactin is essential for the stability of these messages. We have inserted various regions of these genes in the vectors carrying, either a) Chloramphenicol acetyl transferase genes (CAT vectors) or b) neomycin resistant (pSV2 neo) genes, to characterize the DNA sequences through which these hormones induce Alpha-lactalbumin and WP-genes. Primary mammary epithelial cell cultures grown on collagen substratum and several cell lines carrying insulin and hydrocortisone receptors are transfected with the plasmid constructs and the DNA sequences through which these hormones induce these genes are presently being characterised.