In the book Kaplan and Sadock's synopsis of psychiatry:
Clozapine. Clozapine (Clozaril) levels are trough levels determined in the morning before administration of the morning dose of medication. A therapeutic range for clozapine has not been established; however, a level of 100 mg/mL is widely considered to be the minimum therapeutic threshold. At least 350 mg/mL of clozapine is considered to be necessary to achieve therapeutic response in patients with refractory schizophrenia. The likelihood of seizures and other side effects increases with clozapine levels greater than 1,200 mg/mL or doses greater than 600 mg per day or both. Clozapine is a common cause of a leukopenia in psychiatry. When moderate to severe leucopenia develops, clozapine treatment must be interrupted, but patients may be retreated with clozapine in the future.
What is "mL" here?
Sadock, B. J., & Sadock, V. A. (2011). Kaplan and Sadock's synopsis of psychiatry: Behavioral sciences/clinical psychiatry. Lippincott Williams & Wilkins.