NOC (current version 1.5.2.10) is a free program for protein structure model-building, visualization, validation and analysis ... Some features of NOC: Auto-fit residues' side-chain against density data; Solid surface generation and electrostatic potential, hydrophobic force coloring; Potentials calculations; Customize molecular electronic density map contouring; Easy to add/delete HOH, heteroatom Group, residue; Real 3D Stereo Visualizing Support (need a Quad Buffer Graphic Adapter and a Stereo Glasses); Easy to make a animate stream of molecular model; WYSIWYG Labeling method; Easy to compare similar structures; Auto structure checking and hydrogen atoms adding; Predict possible motifs (sites) in the molecule; Auto-determinate secondary structures; Build related molecules and contacts according to molecular crystallographic symmetries; Calculate molecular accessible surface, embedded surface, residue accessibility; Details of Heteroatom interactions in the molecule; Details of Hydrogen bonds in the molecule; Validate structure on the fly; More precise ribbon model representation ; Novel Cartoon model representation ; High quality rendering and many types of output (such as TIFF, PNG, BMP, PS?) ;