Missing residues and incomplete sidechains

From Proteopedia

In about 90% of the Empirical models in the PDB, some residues (amino acids or nucleotides) that were present in the experimental material are absent (have no coordinates) in the empirical model. Local regions of the crystal (for X-ray crystallography), or of the molecules prepared for cryo-electron microscopy, which were disordered are not clearly resolved in the density map used to construct the structure model. Without density to guide where to place these residues, the experimenter omits them from the model. These are called missing residues.