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.