CAMBRIDGE, Mass. (AP) _ Scientists believe they have found nature's sex trigger, a single gene that determines whether a fertilized egg will grow to be a boy or a girl.

Their study suggests that a person's sex is governed by the absence or presence of this gene, which acts as a sort of master regulator.

The gene appears to start a complex chain reaction of hormones that eventually leads to the development of a male. Without the gene, the embryo begins a different pathway and grows into a female.

The gene is located on the Y chromosome, which is part of the inborn genetic library of men but not women. The researchers believe that a virtually identical gene controls sex determination in monkeys, dogs, cattle and all other mammals.

''This clearly is a landmark set of experiments,'' said Nobel laureate David Baltimore, director of the Whitehead Institute for Biomedical Research, where the gene was isolated.

''Although it doesn't tell you anything that you've always wanted to know about the difference between men and women,'' he added, ''it tells you how that difference was first laid down.''

Besides giving important insights into one of the basic processes of life, the research may also provide new clues for coping with infertility and problems of sexual development.

The gene was isolated by Dr. David C. Page and colleagues at the Whitehead Institute, a lab affiliated with Massachusetts Institute of Technology. A report on the work is being published in Thursday's issue of the journal Cell.

Page said his work provides ''a strong circumstantial case'' that the gene really is the sex trigger, but he cautioned that more work will be needed before this is proven conclusively.

Dr. Robert Erickson, who studies sex determination at the University of Michigan, said he is 98 percent sure that Page found the genetic sex switch.

''People have been fascinated from the time of Aristotle about what controlled sex,'' Erickson said. ''To have finally probably found the gene that does it is the answer to a 2,000-year search.''

If Page's discovery holds up, it will also cap a race among labs in several countries. Dr. Peter N. Goodfellow of the Imperial Cancer Research Fund in London said he was several months away from finding the gene himself.

''I think it's the first step in the molecular analysis of sex determination, '' he said of Page's research. ''Obviously there is a lot of work that needs to be done, but he's got the first handle on it. I think it almost certainly is'' the sex-determining gene.

For the first six weeks or so of development in the womb, all fetuses are sexually identical. They can grow to become either boys or girls. Page believes that the newly discovered gene - called TDF, for testis-determinin g factor - rules whether they will grow testes or ovaries and thus be male or female.

Everyone is born with 46 strings of genes called chromosomes. Two of these, called X and Y, are the sex chromosomes. Women have two X chromosomes, while men have an X and a Y.

To find the sex trigger gene, the researchers studied about 60 XX males and XY females. These are ''sex-reversed'' people who appear to be exceptions to the rule. The XX males are sterile, but otherwise they are physically normal men, even though they have the two XX chromosomes of women. XY females appear at birth to be ordinary girls, but they do not mature sexually, and they have the chromosomes of men.

Through a genetic mixup, the XX males actually carry a tiny bit of the Y chromosome on one of their X chromosomes. And the XY females are missing the same portion of their Y chromosome.

Page's research set out to find the piece of Y chromosome that the XX men had and the XY females lacked. Among them was an XX man with only half of 1 percent of a Y chromosome, and an XY female with 99.8 percent of the Y. By analyzing and comparing their genes, he narrowed down the target to one small snippet of genetic material.

Inside this stretch of genetic code, he believes there is one gene that is essential for maleness. Like other genes, it oversees production of a protein, and Page has partially figured out what this protein looks like. He said it resembles a protein seen in frogs that serves to turn other genes on and off.

To prove that they have truly found the sex trigger, the researchers plan to implant the gene into the embryo of a mouse with two X chromosomes and see whether it develops to be a male.

Page said his discovery probably could not be used to intentionally produce male children. Any boys born through shifting the sex trigger gene to an X chromosome would be sterile.