Two major classes of olfactory receptors have been identified in humans:[34]
class I (fish-like receptors) OR families 51-56
class II (tetrapod specific receptors) OR families 1-13
OR6A2, that detects the aldehydes that may make cilantro smell soapy and thus is a compelling candidate gene for the detection of the cilantro odors that give cilantro its divisive flavor.