The "fact" sheet was last updated in 1999. One might expect that subsequent research and medical-scientific discovery would have substantiated the statement to some additional degree.
In 2008 (nine years after NAMI presented their "fact"), Stephen M Stahl published the third edition of his definitive and preeminent, Stahl's Essential Psychopharmacology: Neuroscientific Basis and Pratical Applications. On page 264 of this book, in Chapter 9 Psychosis and Schizophrenia, under the section "Neurotransmitters and circuits in schizophrenia: Dopamine" Stahl states simply, "The biological basis of schizophrenia remains unknown."
Also published in 2008, the third edition of Kaplan & Saddock's Concise Textbook of Clinical Psychiatry weighs in heavily against NAMI's "fact" of nine years earlier. Pages 159-160 contain fourteen separate section-paragraphs, each taking up a specific biochemical factor or possible neuropathology in schizophrenia. Not one of these fourteen sections omits some key admission of inconclusiveness and uncertainty. E.g., "...biochemical and pharmacological data bearing on this proposal are inconclusive", "The ... dopamine hypothesis of schizophrenia...", "... whether an active pathological process is continuing to evolve in schizophrenia patients is uncertain."
Stahl's and Kaplan & Saddock's are authoritative, mainstream medical-psychiatric sources. They incorporate the understanding resulting from almost a decade of additional research since NAMI-GC's "fact" sheet was last updated. There are of course, many more authorities, and guess guess what? Not one of them validates the utter falsehood quoted above from NAMI-GC's web site. There is no known biological basis for any mental illness.
Why does NAMI persist in defrauding the public about mental health?
Better yet, why does the State of Illinois persist in offering NAMI's deceptive propaganda as "education" to the public? (See my posts of December 18 and 21.)