TOB1 is a member of the tob/btg1 family of anti-proliferative proteins that have the potential to regulate cell growth. When exogenously expressed, this protein supresses cell growth in tissue culture. The protein undergoes phophorylation by a serine/threonine kinase, 90kD ribosomal S6 kinase. Interactions of this protein with the v-erb-b2 erythroblastic leukemia viral oncogene homolog 2 gene product p185 interferes with growth suppression. This protein inhibits T cell proliferation and transcription of cytokines and cyclins. The protein interacts with both mothers against decapentaplegic Drosophila homolog 2 and 4 to enhance their DNA binding activity. This interaction inhibits interleukin 2 transcription in T cells.