Two HIV-infected patients in Boston who had bone-marrow transplants for blood cancers apparently have been virus-free for weeks since their antiretroviral drugs were stopped, researchers at an international AIDS conference announced Wednesday.

The patients' success echoes that of Timothy Ray Brown, the so-called "Berlin patient" who has shown no signs of resurgent virus in the five years since he got a bone-marrow transplant from a donor with a mutation conferring resistance to HIV.

The Boston cases, as with Brown's, are of no practical use to the 34 million people who have HIV but neither blood cancer nor access to premier hospitals.

But AIDS experts still find the Boston cases exciting because they are another step toward a cure and offer encouragement to future projects to genetically re-engineer infected patients' cells to be infection-resistant. At least two teams are experimenting with variants on this idea, said Dr. Steven G. Deeks, an AIDS researcher at the Medical School of the University of California, San Francisco.

Dr. Franoise Barr-Sinoussi, a discoverer of the virus that causes AIDS and the president of the International AIDS Society, called the findings about the Boston patients "very interesting and very encouraging."

For example, no AIDS expert, including the doctors from Brigham and Women's Hospital in Boston following the two patients, is using the word "cured" to describe their status.

One patient stopped taking antiretroviral drugs seven weeks ago. For the other, it has been 15 weeks.