The number of three-dimensional (3D) structures available has been growing rapidly in the last two decades. Nevertheless, not all structures are available and some are quite scarce, despite the recent technological improvements (e.g. membrane proteins). For molecules whose structures are not yet available in databases, theoretical models can still be derived. There are several approaches available in the structure modeling field, but they mainly fall into three categories: comparative modeling methods, ab initio, and threading methods. From these approaches, comparative modeling, most known as homology modeling, remains the most accurate and widely used. (Sample)