Compositions and methods are provided for the clinical assessment, diagnosis, and treatment of multiple sclerosis. The compositions of the invention are molecules related to the 21.5 kDa fetal isoform of human myelin basic protein, and include nucleic acids and polypeptides. The nucleic acid molecules of the invention are useful in the efficient production of modified and unmodified 21.5 kDa myelin basic protein polypeptides, such polypeptides being useful for assaying T cells for responsiveness to myelin basic protein epitopes. The polypeptides of the invention are also useful as therapeutic agents that act by inducing T cell responses, including apoptosis, as a means of treating multiple sclerosis.