The arrival of the AIDS epidemic and genetic engineering coincided, and the result has been a frenetic wave of research that has allowed researchers to learn more about the virus faster than any other in history.

Reports in Science and Nature magazines this week describe the discovery of a gene in the virus that is crucial for infection.

The AIDS virus contains only a handful of genes; only a few are required for its simple but deadly mission. While one gene makes the outer coat of the virus and another makes the structural core, there has been one mysterious gene, called "sor," whose function was unknown.

When researchers splice out this gene, the virus has great difficulty in attaching to cells and injecting itself into them. The virus is 1,000 times less infective without sor, according to the Science paper, whose chief author is Klaus Strebel of the National Institute of Allergy and Infectious Diseases. The research team, which reported its results in Nature, was led by Amanda Fisher of the National Cancer Institute.

Unfortunately, the virus has another mode by which it can be passed from cell to cell without using its regular infectious apparatus. One infected cell can sidle up to an uninfected one, link up and transfer its load of infestation through the cell wall.

The discovery that sor is the key to infection, however, will give researchers another tool to determine the prime mode of passing acquired immune deficiency syndrome -- through infection or cell-to-cell transmission -- by allowing them to distinguish between the two.