In order to determine whether there are detectable changes in plasma proteins or peptides accompanying the emergence of viremia, an initial mass spectrometry-based screen was performed on three plasma donor panels (Fig. 1B) . Analysis of the longitudinal MALDI-TOF data revealed mass peaks that were elevated at viremic time points ( Fig. 2A) . One mass peak with a molecular mass of 2178 Dalton [M+H] + was found to be considerably elevated in HIV-1-positive plasma ( Fig. 2A) . Sequencing by MALDI-TOF/TOF and LC-MS/MS identified this mass as peptide 86-105 derived from A-SAA ( Fig. 2B and Fig.  S1A) . A semi-quantitative analysis of mass peak intensities of the 2178Da [M+H] + peptide mass revealed that this peptide was elevated coincident with the increase in viremia, and in 2 of the 3 subjects, immediately prior to the detection of viremia (Fig. 2C) , suggesting that A-SAA protein levels are elevated at these times. A second mass peak with a molecular mass of 2213 Dalton [M+H] + was identified as peptide 960-979 of complement C3 (Fig. S1B) . This peak was also elevated prior to as well as during viremia (Fig. 2C ).