61.   But embryonic cells seem less antagonistic to the immune system than are ordinary cells.

62.   But Eddy cautioned that it could just be that cells are sloppy, transcribing far more DNA into RNA than they use.

63.   But Gibbons said the researchers do not know if the cells will be of any greater therapeutic benefit than those already in existence.

64.   But if cells are stimulated repeatedly, something else happens, George said.

65.   But others say that while the memory cells may be a source of the virus, they cannot be the whole story of the HIV reservoir.

66.   But since guards patrol outside the cell block at night and individual cells are easily unlocked, the inmates had plenty of time to flee.

67.   But scientists say those cells may not be as useful to scientific research as embryonic stem cells because they have a limited ability to transform into specialized cell types.

68.   But such cells are at the center of an ethical controversy, with opponents saying it is immoral to use embryos for medical purposes.

69.   But muscle cells are among the few in the body that actually take up so-called naked DNA, genes that are not packaged in viruses or wrapped in membranes.

70.   But these cells are rare and difficult to obtain.