Isaria sinclairii is a species of entomopathogenic fungus mostly infecting the underground nymphs of cicadas.

It produces myriocin, from which the synthetic drug fingolimod, a treatment for multiple sclerosis, was developed.

[3] Isaria sinclairii is a fungus which attacks insects, including cicada larvae.

[7][8] A sphingolipid derivative produced by I. sinclairii, myriocin, was discovered to have powerful immunosuppressive properties, a function of the way the fungus attacks living insects.

[10][8] Under its trade name Gilenya, fingolimod was approved by the FDA in 2010 as the first oral drug for treating the autoimmune disease multiple sclerosis.