The finding -- certain to add fuel to the already heated debate over gay rights -- supports earlier studies which suggested that inherited genetic factors at least play a role in determining sexual orientation.

``Being gay is not simply a choice or purely a decision. People have no control over the genes they inherit and there is no way to change them,'' said the study's lead author Dean Hamer, chief of the cancer institute's Section on Gene Structure and Regulation.

Hamer and his colleagues began by studying the family histories of 114 gay men and found more homosexual brothers, uncles and male cousins than would be expected in the general population. Some families had three generations of homosexual relatives.

``Since the uncles and cousins aren't raised in the same household but share genetic information, that suggested there was something inherited going on,'' Hamer said in an interview.

Following up on that suggestion, Hamer studied the DNA from 40 pairs of homosexual brothers and found 33 of them shared genetic markers on the X chromosome in a region known as Xq28.

The X chromosome is one of two sex-determining chromosomes; it is always inherited from mothers. Genes are arranged along 46 chromosomes, each consisting of tiny coils of DNA, deoxyribonucleic acid, which carries the instructions to manufacture a particular body substance.

There was no such similar sharing in the same region among heterosexual men. The researchers have not yet compared the homosexuals' genetic information to the other group.

``We expect that this region will be important for both heterosexual and homosexual development -- that there will be a very small and subtle difference'' between the genes of each group, Hamer said.

However, the finding does not explain all homosexuality. Seven out of 40 pairs of homosexual brothers studied did not have the common genetic factor, the researchers said.

The research, published in the journal Science, was conducted as part of the National Cancer Institute's study of cancers such as Kaposi's sarcoma which afflict unusually large numbers of homosexuals.

Further study is being conducted to determine whether a similar genetic link occurs in families of homosexual women. Hamer said he also hopes to identify the specific gene involved in sexual orientation.

Gregory King, spokesman for the Human Rights Campaign Fund, a gay and lesbian activist group, said he hoped Hamer's study would ``help Americans understand that most lesbian and gay people do not choose their sexual orientation.''

But King also expressed concern that anti-homosexual activists could misconstrue the cancer institute study.

``There will be concern among people who are lesbian and gay that this discovery will be misused to suggest that (homosexuality) is something that needs to be 'corrected,''' King said.