Large-scale analysis directly at the protein level holds the promise of uncovering features not apparent or present at the gene level [1-3]. Although mass spectrometry (MS)-based proteomics can now identify and quantify thousands of cellular proteins in large-scale proteomics experiments, much of the peptide information contained in these experiments remains unassigned [4]. Here, we use such information to discover a previously unreported mechanism creating altered protein forms. Linker histones H1 and high-mobility group (HMG) proteins are abundant nuclear proteins that regulate gene expression through modulation of chromatin structure [5-8]. In the high-resolution MS analysis of histone H1 and HMG protein fractions isolated from human cells, we discovered peptides that mapped upstream of the known translation start sites of these genes. No alternative upstream start site exists in the genome, but analysis of Expressed Sequence Tag (EST) databases revealed that these N-terminally extended (ET) proteins are due to in-frame translation of the 5' untranslated region (5'UTR) sequences of the transcripts. The new translation start sites are created by a single uridine insertion between AG, reflecting a previously unreported RNA-editing mechanism. To our knowledge, this is the first report of RNA-insertion editing in humans and may be an example of the type of discoveries possible with modern proteomics methods.