1 | INTRODUCTION
CASP is a well-established experiment for tracking progress in computational methods for calculating protein structure from amino acid sequence and related challenges1-4. While the core of CASP has remained unchanged since its inception, the scope changes frequently, with some categories becoming obsolete while new ones are introduced. Following the major advances in single protein structure modeling seen at CASP14, three categories were discontinued for round 15: contact prediction, refinement, and third-party model accuracy estimation for monomeric targets. At the same time, discussions among the organizers and advisory board members as well as a survey of the modeling community established interest in the new categories of RNA structure, protein-ligand complexes, accuracy of oligomeric structures and their interfaces, and ensembles of alternative conformations. CASP launched pilot experiments in all these areas, exploring, where possible, collaborations with groups involved in the existing or recently discontinued related experiments. In CASP15, the new categories have yielded interesting results, and their assessment is discussed in dedicated papers in this journal special issue5-8. The main goal of this paper is to address technical issues of introducing the new categories in CASP and serve as a point of reference for format specifications and details on how the experiment was conducted.