MICROBIOLOGY. For the article “Purported nanobacteria in human blood as calcium carbonate nanoparticles,” by Jan Martel and John Ding-E Young, which appeared in issue 14, April 8, 2008, of Proc Natl Acad Sci USA (105:5549–5554; first published April 2, 2008; 10.1073/pnas.0711744105), the authors note that on page 5550, right column, line 3, “P:Ca ratio” should have appeared as “Ca:P ratio.” This error does not affect the conclusions of the article.