Millitomes provide guidance on how to partition an organ, often a complete organ. Millitomes can include either a 3-dimensional (3D) mold that physically holds an organ for partitioning, or a drawing of an organ that is used as a guide by a surgeon when they partition an organ free hand, or both. Using a millitome ensures that tissue sample locations show correctly in the Exploration User Interface. The 3D millitome (from Latin mille, meaning “thousand,” as in millimeter, and the Greek temnein, meaning "to cut") is a device designed to hold a freshly procured organ and facilitate cutting it into many small tissue blocks of well defined size for usage in single cell analysis and Human Reference Atlas construction. A millitome has discrete, equally placed cutting grooves in both the x and y directions to guide a carbon steel cutting knife. The millitome is used to produce uniformly sized slices or cubes of tissue material that can be registered to organs from the 3D Reference Object Library. The procedures outlined here describe how to print and use a millitome and how to ensure that spatial locations for each slice or cube are retained for use in atlas construction. The primary audience for this SOP is the wet bench scientist who will be using a millitome to cut and register multiple tissue blocks from a single organ.