Chasing the Ghost Proteome in the Dark matter.

Emerging evidence shows that translation from non-canonical open reading frames (ORFs) produces a diverse set of biologically active proteins. These ORFs reside in 5' and 3' untranslated regions, long non-coding RNAs, overlapping frames within annotated genes (dual coding), pseudogenes, and can initiate at non-AUG start codons. The resulting products, variously termed microproteins, small proteins (smPROTs), small ORF-encoded peptides (SEPs), and alternative proteins (AltProts), modulate fundamental cellular processes, including metabolic flux and epigenetic regulation. We consolidate these entities under the umbrella of the ghost proteome, a functional proteome arising from the genome's presumed "dark matter." This concept is distinct from the dark proteome, which refers to regions of canonical proteins lacking structural, functional, or experimental annotation and is not necessarily derived from non-canonical loci. Recognizing the ghost proteome expands the boundary of what is considered protein-coding, demands harmonized nomenclature and database integration, and motivates systematic discovery and functional characterization. By reframing sequences once dismissed as non-coding or "junk," the ghost proteome compels a re-evaluation of genome annotation and reveals new opportunities to interrogate biology and disease.