The invention features methods diagnostic of a psychotic disorder such as bipolar disorder or schizophrenia. The methods include obtaining a cell sample from a subject, subjecting a cell from the sample to stress (e.g., nutrient stress), and measuring nucleic acid or polypeptide expression in the cell, where an alteration in expression is indicative of the subject having or being at increased risk of developing a psychotic disorder. The invention also features prognostic monitoring methods for subjects having a psychotic disorder, useful in determining the progression of a psychotic disorder in a subject or the effectiveness of a therapy.