Reticulocalbin-2 (RCN2) as a Context-Dependent CREC-Family Modulator: Gene Expression and Regulation, Molecular Biology, Disease Relevance, and Translational Challenges

Reticulocalbin-2 (RCN2), encoded by the human RCN2 gene (HGNC:9935) and historically identified as E6-binding protein, is a CREC-family EF-hand Ca2+-binding protein. Although first characterized as an endoplasmic reticulum (ER)-associated protein, current evidence extends RCN2 biology to splice-variant and localization complexity, protein interactions, regulatory and signaling networks, extracellular activity, cancer, non-malignant disease, and the RCN2-derived hormone Raptin. This review synthesizes the fragmented and methodologically uneven RCN2 literature within an evidence-stratified framework. Mechanistic evidence, including direct biochemical assays and RCN2-directed functional studies, supports selected interaction and signaling relationships, whereas omics, database-derived and single-context observations remain hypothesis-generating. Across disease settings, RCN2 is best interpreted as a context-dependent modulator rather than a uniform oncogenic driver, validated biomarker or established therapeutic target. Translational progress will require molecular-form-specific assays, disease-specific validation and clinically grounded models.