“…In X-ray crystallographic experiments, and thus in many of the three-dimensional structures deposited in the PDB, the loops and ends of the polypeptide chains and certain residues in the middle portion of the chain are not observed owing to poor electron density (Sumathi et al, 2006;Saravanan et al, 2010;Djinovic Carugo & Carugo, 2015;Gurusaran et al, 2016). Thus, these regions will not be available in the atomic coordinates file submitted to the PDB and they are referred to as 'missing regions' (Ananthalakshmi, Kumar et al, 2005;Ananthalakshmi, Samayamohan et al, 2005;Nagarajan et al, 2012;Santhosh et al, 2019). Depending on the location of the missing regions in a polypeptide chain, they can be classified into three categories: 'N-terminal' at the beginning of the polypeptide chain, 'C-terminal' at the end of the polypeptide chain, and 'Middle' anywhere between N-terminal and C-terminal.…”