UniProt data pruning and the growing invisibility of the dark proteome.

The protein resource UniProtKB has undergone a major, multi-stage reorganization of its protein space, implemented from release 2025_04 through release 2026_02. Analysis of UniProtKB and UniParc records accessed on 20 January 2026 showed that 125.9 million entries were last present in UniProtKB in 2025 and are now represented in UniParc, while ~200.0 million entries remained in UniProtKB at the access date. Of these, 58.9 million were designated for removal in release 2026_02, whereas 141.2 million were classified as retained. The reorganization retains all reviewed UniProtKB/Swiss-Prot entries (the expert-curated subset of UniProtKB), along with proteins from selected reference proteomes and unreviewed entries with additional biologically important data, including experimentally determined three-dimensional structures. The expected benefits include reduced redundancy, easier navigation, and a protein set that is more tractable for annotation and routine searches. However, the reorganization may reduce the visibility of protein sequences from uncultured microorganisms and metagenomic surveys, phylogenetically underrepresented lineages, particularly many archaeal groups, proteins belonging to the so-called "dark proteome," and strain-level variants of well-studied species such as Escherichia coli. We argue that the main consequence is not the loss of the sequences themselves, because affected proteins remain recoverable through UniParc and related resources, but a reduction in their discoverability, annotation context, and accessibility within routine workflows. These effects are likely to be most important for biodiversity-oriented analyses, retrospective reuse of older datasets, studies of specialized protein families, and downstream pipelines that rely on UniProtKB as an annotation hub. As UniProt shifts toward a more explicitly reference-proteome-centered model, a critical question arises: What are the implications of UniProtKB contraction for the representation of evolutionary diversity across the tree of life, particularly in under-sampled and uncultured lineages?