The Expanding Chemical Alphabet of the Human RNome

More than 180 chemically distinct RNA modifications – the epitranscriptome – have now been reported across all kingdoms of life and all forms of RNA. Given the emerging evidence for critical roles played by RNA modifications in many diseases, it is imperative to identify the function of all RNA modifications in the nearly 250,000 types of RNA in human cells – the human RNome. Here we take a step toward this goal by reviewing the literature to date and assembling a catalog of all established or putative human RNA modifications. While the identities and locations of many human RNA modifications are well established, emerging technologies are revealing new ones as well as old ones in unexpected locations, with low abundance and poorly characterized structures hampering their validation. To this end, we introduce an analytical validation framework that scores each reported human RNA modification for chemical identity and transcriptomic localization and then assigns it to one of three confidence tiers: Authenticated, Provisional, or Putative. Applying this framework, we critically re-examine the primary literature underlying 24 recently reported additions to the human epitranscriptome, evaluating the mass spectrometric, spectroscopic, and sequencing-based evidence supporting each as well as discussing their emerging biological roles and disease associations. We further extend this scoring system to modifications previously summarized in reviews and databases but not yet subjected to this level of scrutiny, compiling a comprehensive, tier-ranked inventory of the human epitranscriptome as of July 2026. Together, this catalog and framework provide a benchmark for evaluating future epitranscriptome discoveries, mapping the human RNome, and for derisking the translation of validated modifications as diagnostic and therapeutic targets.