In the FBDD process, a library of fragments is first screened against a given protein target in order to identify potential binders. These fragments usually adhere to the so-called "rule of three" , having a molecular weight ≤ 300 Da, ClogP ≤ 3, a number of hydrogen bond donors ≤ 3 and a number of hydrogen bond acceptors ≤ 3. From this initial screen, binders are then identified through structural (e.g. X-ray crystallography, nuclear magnetic resonance (NMR)), biochemical, or biophysical (e.g. NMR, surface plasmon resonance (SPR), isothermal titration calorimetry (ITC)) characterisation . Confirmed binders are then optimised, heavily relying on structure activity relationships to create bespoke high affinity binders. Optimising strategies either concentrate on improving the affinity of a single binder through fragment growing, or by combining multiple fragments occupying distinct binding sites through linking and merging decisions .