ability to multiply. As an added attraction, bioengineers can ensure that the protein products expressed by added genes are deposited in the milk of mammals or the eggs of hens, making the chemicals easy to harvest and process with little or no detrimental effect on the animals. Here are some of the products already being developed:

• Lysozome is an antibacterial agent that makes up about three to four percent of a normal egg white. Researchers are manipulating the lysozome gene to increase the volume of antibiotic produced and to make lysozome effective against a wider range of bacteria.
• Egg yolk normally contains antibodies that are deposited by the hen to protect the embryo from infection before its own immune system develops. The variety of antibodies can be customized by first immunizing hens with particular antigens. This strategy can now be taken one step further by making transgenic hens. Given genes from other species, these hens will lay eggs with antibodies specific to diseases of, say, pigs, cattle, or people.
• Female mammals regularly produce large quantities of protein in their milk. Scientists can modify the milk content by giving the animals added genes encoding various therapeutic proteins. After the milk is collected, the desired proteins are isolated and purified for use. Products already made in this way, using milk from cows, pigs, and sheep, include human lactoferrin (a good source of iron for babies), antitrypsin (a drug used to treat emphysema), human protein C (needed for proper blood coagulation), collagen (for tissue repair), and fibrinogen (a tissue adhesive).