The Tob/Btg family of proteins consists of a large number of members. These proteins have a common domain in their amino terminal end and may have anti-proliferative activity in various cell types. The Tob protein was identified in a search for molecules that interact with the receptor tyrosine kinase ErbB2. Active ErbB2 has a negative effect on the anti-proliferative activity of Tob. However, Tob is phosphorylated on serine and threonine residues but not on tyrosine, suggesting that active ErbB2 activates a Ser/Thr kinase that phosphorylates Tob. Unphosphorylated Tob suppresses expression of cyclin D1. It was shown that active p90Rsk1 kinase (known to be activated by protein-tyrosine kinase receptor) phosphorylates Tob on serine and threonine residues in vitro. In addition, Erk1/Erk2 MAP kinases phosphorylate Tob in vivo and in vitro, resulting in suppression of the anti-proliferative activity of Tob. Homozygous Tob knockout mice develop greater bone mass resulting from increased numbers of osteoblasts. Furthermore, it has been shown in osteoblasts, that upon BMP2 (bone morphogenetic protein) activation, Tob associates with receptor regulated Smads (Smad 1, 5, and 8). Thus, osteoblast proliferation and differentiation is negatively regulated by Tob protein through the Smad proteins.