With recourse to Greek mythology, it is customary to refer to hybrids of humans and animals as "chimeras". However, in biology the term has a broader definition. Here, a chimera is understood to be a biological entity consisting of cells derived from at least two different zygotes (i.e. a cell resulting from the fusion of germ cells) and therefore composed of genetically distinct cells. Because this definition does not presuppose that the various cells must originate from organisms of different species, recipients of donor organs are already chimeras in the biological sense after successful transplantation. A hybrid, on the other hand, is a biological entity that combines characteristics of different, previously independent species or classes. This means that all cells have the same genotype (e.g. mule and hinny, which are the product of crossing different species). Because the biological differentiation between chimeras and hybrids is irrelevant for most ethical questions concerning hybrids of humans and animals, the bioethical literature has proposed that these be collectively referred to as "chimbrids".