Figure : Left: Functional groups can be interconverted by a simple cut-and-join crossover. The general position (primary, secondary, tertary, quaternary) of a functional group can be changed by means of crossover operations that replace a hydrogen atom by a larger fragment (dotted arrows). Right: Cut-and-join crossover involving hydrogens enable access to simple reactions. desirable properties. Most importantly, functional groups can be easily transferred between molecular scaffolds. To this end, cuts of size 1 or 2 are sufficient, see Fig. . Labels, i.e., atom types, with the same valency can be replaced by cuts of size deg(x). Moreover, hydrogen atoms are readily exchanged for larger molecular fragments, provided there is a source graph from which the desired fragment can be cut. Simple reactions such as ether formation, esterification, etc., thus have their direct counterpart in cut-and-join crossover, see Fig. .