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Transgenic mice produced in this investigation contained the proximal promoter elements, the genomic coding sequence and the 3' flanking region of bovine β-casein and the 5' regulatory elements of bovine α-lactalbumin. Specifically, the gene construct included the TATAA box, CAAT box and polyadenylation signal of bovine β-casein in addition to the 5' regulatory elements of the bovine α-lactalbumin gene. Four lines of transgenic mice were generated; they expressed bovine β-casein in their milk at concentrations of up to 10 mg mL-1. The milk from the transgenic mice tended to be very viscous and a proportion of these mice stopped lactating before the normal onset of involution. The time at which the abrupt involution occurred was directly correlated to the amount of β-casein produced. Mice secreting approximately 10 mg mL-1 of β-casein in their milk stopped lactating approximately 1-3 days after parturition while mice expressing 1-3 mg mL-1 stopped lactating around days 10-18. The high expressing mice appeared to lactate normally on day 1 of lactation, but on day 3 the mammary gland morphology revealed alveoli and ducts which were very distended and contained milk components. However, the viscous milk solution could not be removed from the gland at this stage of lactation. There were also a number of alveoli that appeared to have gone through the process of involution by day 3 of lactation.


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