There is no single definition for the term "bioidentical hormone replacement therapy" (BHT); it is generally used to refer to 17β-estradiol, but other uses include plant-based or compounded estrogen products that blend estradiol with estriol and sometimes with estrone. A "bioidentical hormone" is defined as a molecule identical to a hormone produced by the human body (though not all allegedly bioidentical hormones sold by custom-compounding pharmacies are necessarily molecularly identical to endogenous humans). The FDA considers "BHT" as currently used by BHT advocates to be a marketing term, not a scientific term, and does not recognize its use. The meaning of "plant-derived" has also been attached to the term "bioidentical", and it may also mean that the hormones are "natural"; throughout the 1990s plant-derived, compounded hormones were referred to as "natural hormone therapy". However, the term "natural" can be applied to all products where the principal ingredient originates from an animal, plant, or mineral source, and both bioidentical and nonbioidentical hormones can be produced from the same plant sources.