By analyzing TCR Vβ usage in two mouse strains that spontaneously develop autoimmunity and in the MOG-EAE model, we provide evidence that prenatal betamethasone treatment, by changing the peripheral T cell repertoire, modifies the autoreactive T cell pool, and this may lead to unpredictable variations in the course of autoimmune disease, depending on which T cell clonotypes are affected. The gene discussed is MOG; the disease is autoimmune disease.