These constraints could include: desired biological effect (primary constraint), drug-likeness, pharmacokinetic properties, toxicity or chemical feasibility (secondary constraints). The desired biological effect is considered the primary constraint for the reason that all programs contemplate this objective. Most recent software considers fragments above atoms as building blocks. Fragments' databases come from different sources including external inputs as catalogs or fragment-like compounds. Databases can also be constructed from virtual fragmentation of complete drug molecules to increase the probability of obtaining a drug-like molecule with synthetic accessibility.