Disclosed is a FAST (Flexible Accelerated STOP TetO-knockin) system, an efficient method for manipulating gene expression in vivo to rapidly screen animal models of disease. This invention further discloses a single gene targeting event yielding 2 distinct knockin mice—STOP-tetO and tetO knockin—which permit generation of multiple strains with variable expression patterns: 1) knockout, 2) Cre-mediated rescue; 3) tTA-mediated misexpression; 4) tTA-mediated overexpression; and 5) tTS-mediated conditional knockout/knockdown. Using the FAST system, multiple gain- and loss-of-function strains can therefore be generated on a timescale not previously achievable. These strains can then be screened for clinically-relevant abnormalities. The flexibility and broad applicability of the FAST system is demonstrated by targeting several genes encoding proteins implicated in neuropsychiatric disorders: Mlc1, Neuroligin 3, the serotonin 1A receptor, and the serotonin 1B receptor.