21.   One way to do that would be to add genes to cells that force the cells to produce the drugs every time they made milk proteins.

22.   Or they add genes to these same rodents and observe the results.

23.   Others are trying to add genes to healthy stem cells of cancer patients, to make them resistant to chemotherapy.

24.   PPL, which sponsored the work, refused to say which human gene was added to the sheep until it publishes the work in a scientific journal.

25.   Scadden and others, for example, want to add genes to stem cells that would make them resistant to the human immunodeficiency virus, which causes AIDS.

26.   Several explanations are emerging for how to generate extra complexity other than by adding more genes.

27.   Several years ago, investigators discovered that an inactivated cold virus could add normal genes to cultured cells of patients with cystic fibrosis.

28.   Scientists would grow pig cells in a laboratory and add genes that would make their surface proteins identical to proteins that coat human cells.

29.   Still others want to treat diseases like sickle cell by adding genes to marrow stem cells.

30.   That makes it possible to add genes to them.