1.   Biologists recently got a taste of immortality when they found that human cells kept in culture would grow and divide indefinitely if their telomeres were artificially lengthened.

2.   But human cells have a gene whose product, called telomerase, can lengthen telomeres.

3.   It continues to shorten with each cell division until it finally becomes so short that it is not binding enough proteins to inhibit the enzyme that lengthens the telomeres.

4.   The theory said that if scientists could lengthen the telomeres of the elderly, they might be able to restore youth.

5.   This tempting analogy became much strengthened when biologists found that all human cells possess a gene whose product, named telomerase, can lengthen the telomeres.

6.   When the number of proteins exceeds a critical value, an enzyme that lengthens telomeres cannot work.