A method of purifying Mycoplasma arthritidis mitogen (MAM) to electrophoretic and sequence homogeneity is disclosed. A preparation of MAM purified according to this method was used to determine the sequence of the N-terminal 54 amino acids of MAM. A synthetic peptide consisting of amino acids 15-32 inhibited MAM-induced cell proliferation in vitro. The sequence of the N-terminal 54 amino acids was reverse translated, nucleotide probes were designed therefrom, and the MAM gene was selected from a genomic library. The MAM gene was sequenced and found to be contained on a 1107 bp DNA fragment. The primary translation product contains a 39 amino acid signal sequence and a 213 amino acid mature MAM (molecular weight 25,294). Amino acid sequence comparisons of MAM to bacterial and murine tumor virus superantigens showed regions of conservative sequence homology, including the region capable of inhibiting cell proliferation. Sequence homologies to HIV and other retrovirus proteins and to certain regulatory proteins were also detected. Strategies for blocking or immunizing against certain diseases, including autoimmune diseases, are disclosed.