The study looked at how HIV-1 mutates and affects its ability to replicate in the body. By analyzing genetic data from untreated HIV-1 infections, researchers found that mutations occur at a rate of 1.2 x 10^-5 per site per day. Most mutations that change the virus's genetic code make it harder for the virus to replicate. Half of these harmful mutations have a big impact on the virus's ability to survive, while others have little effect. The cost of harmful mutations is higher in certain parts of the virus's genetic code, like RNA structures and regulatory regions. These findings suggest that the harmful effects of mutations are consistent across different patients and play a big role in the diversity of HIV-1.