Stereo views of the active site of CD38. Top panel. A close-up surface view is shown with a molecule of nicotinamide mononucleotide (NMN) bound to the active site pocket. Color coding for its atoms: green, carbon; blue, nitrogen; yellow, phosphorus; red, oxygen. Hydrogen atoms are not shown for clarity. The van der Waal’s surface are colored transparent white, allowing the critical residues to be seen. Color code for atoms of the residues: black, carbon; cyan, nitrogen; red, oxygen; white, hydrogen. Bottom panel. Critical residues and NMN (yellow) are shown. Numbers are distances in Angstroms. Glu146 is colored cyan; Asp155, magenta; Asp147, light gray; and dark gray for Trp125, Trp189, and Gly226.