of the molecule is complex and expensive. It is also unstable in the environment due to degradation by light and heat. The apparently mistaken association of aminopterin with its use as a rodenticide likely dates back to a 1951 patent issued to the American Cyanamid Company (then the holding company of Lederle Laboratories) that is commonly cited by a variety of reference textbooks. Aminopterin has a single-dose LDLo of 2.5 mg/kg when orally administered to rats. Aminopterin is widely used in selection media (such as HAT medium) for cell culture, particularly in the development of hybridomas, which secrete monoclonal antibodies. Implication in 2007 Menu Foods recall On March 23, 2007, ABC News reported that aminopterin was the chemical linked to the 2007 Menu Foods pet food contamination incident. The incident resulted in a massive recall of the affected foods. The link to aminopterin was confirmed by New York State Agriculture Commissioner Patrick Hooker and Dr. Donald Smith, Dean of Cornell University's College of Veterinary Medicine, in a statement released on the same day. On March 27, the ASPCA Animal Poison Control Center expressed concern that the problem may not yet be fully understood and that other contaminants may be involved, noting that "clinical signs reported in cats affected by the contaminated foods are not fully consistent with the ingestion of rat poison containing aminopterin". On March 30 it was widely reported that the United States Food and Drug Administration had found melamine in wheat gluten that was used in the pet foods in question. These same reports stated that the FDA had failed to find evidence of aminopterin in the wheat gluten. Tests at the University of Guelph in Ontario, Canada detected aminopterin in some pet food samples, but only in concentrations of parts per billion or parts per trillion, amounts too low to cause the symptoms seen. Exposure and treatment Symptoms of exposure in humans include: nausea vomiting anorexia weight loss chills fever stomatitis – inflammation of the oral mucosa pharyngitis – inflammation of the pharynx erythematous rashes – red rashes on the skin hyperpigmentation – increased pigmentation associated with cleared psoriatic lesions gastrointestinal hemorrhage renal failure – in high doses necessarily involving concomitant leucovorin rescue abortions in pregnant women Supralethal doses of aminopterin may be rescued with the antidote Leucovorin (also known as folinic acid), a reduced form of folic acid which bypasses dihydrofolate reductase, the enzyme inhibited