(3 and 3 possibilities). This yields 17 possible modulator combinations. There are 18 (=2*3*3) if neutral modulator type is also included.

For all practical considerations, these combinations can be generalized only to 5 classes and 1 neutral:
* positive allosteric modulators (PAM) increase agonist affinity and/or efficacy. Clinical examples are benzodiazepines like diazepam, clobazam and chlordiazepoxide,