Cn3D is NCBI's 3D structure viewer. Cn3D is a visualization tool for biomolecular structures, sequences, and sequence alignments. What sets Cn3D apart from other software is its ability to correlate structure and sequence information: for example, a scientist can quickly find the residues in a crystal structure that correspond to known disease mutations, or conserved active site residues from a family of sequence homologs. Cn3D displays structure-structure alignments along with their structure-based sequence alignments, to emphasize what regions of a group of related proteins are most conserved in structure and sequence. Also included are custom labeling features, high-quality OpenGL graphics, and a variety of file exports that together make Cn3D a powerful tool for literature annotation. Cn3D is typically run from a WWW browser as a helper application for NCBI's Entrez system, but it can also be used as a standalone application.