Methyl 4-phenylpyridinium (MPP+) is a transported substrate for OCT 1, 2, and 3.13 Tetramethylammonium (TMA) and tetraethylammonium (TEA) are model cations. Spermidine (a7) and spermine (a10) are naturally occurring polyamines. Polyamine analogs (b–e) are classified according to the hydrophobicity of the end groups (letters, in ascending order); within each class, compounds are ordered according to the number of carbons between end groups. Note that decylmethylammonium only has one charged group, but has been designated c10-b to indicate its structural resemblance to decamethonium, c10-a.