In two recent papers, Mao et al. (1, 2) present the trimeric structure of the HIV-1 gp160 trimer at 11and 6-Å resolutions, respectively. The authors repeatedly emphasize their “reference-free,” “gold standard” methodology, but in neither of their papers do they state how the particles were selected from the electron micrographs. They do state (figure S3 in ref. 1): “Each row shows a sequence of...