In 2009, a Collins County, Texas, jury sentenced Philippe Padieu to 45 years in prison for aggravated assault with a deadly weapon – having sex with a series of women and not telling them he had HIV. An important part of the evidence that identified him as the source of the women's infection came from experts at Baylor College of Medicine ( and The University of Texas at Austin (

In a report that goes online today in the Proceedings of the National Academy of Sciences (, Dr. Michael Metzker (, associate professor in the BCM Human Genome Sequencing Center (, Dr. David Hillis ( of UT Austin and their colleagues, describe how they identified Padieu and a man in Washington State in two different cases as the sources of HIV infection to multiple female partners.

"We were blinded in the study," said Metzker. That means they did not know which sample came from the men accused in the crimes and which came from the women who had become infected with HIV.

In determining the source of the infection, they relied on the "bottleneck" that occurs during HIV transmission.

"Within a given person, there is not just one strain but a population of strains because HIV mutates all the time when it makes new virions (viral particles)," said Metzker. "During transmission, however, there is a genetic bottleneck in which only one or two viruses get transmitted to the recipient."

"As many as 75 percent of HIV infections results from a single virus," said Metzker. That means that even though HIV changes in the body, there is a single virus that is the "ancestor" or progenitor of all those viruses.

"Phylogenetic analysis allows us to reconstruct the history of the infection events," said Hillis, professor at UT Austin. "We can identify the source in a cluster of infections because some isolates of HIV from the source will be related to HIV isolates in each of the recipients."

In comparing DNA sequences, Metzker and his colleagues looked at two gene regions of the virus. They are known as env and pol. Comparing these sequences in the different case samples and using mathematics to model evolutionary change, they were able to identify in each case that the viral sequences from case samples were related. More important, they could identify which case sample was the source of the infection.

Only after the scientist had done all the sequencing and analysis did the District Attorneys' offices break the code. In each case, the sample that they thought was the source of the infection came from the man accused of transmitting the virus to the unsuspecting women.

"This is the first case study to establish the direction of transmission," said Metzker.