It is well known that loss of tumor suppressor genes and more generally of antiproliferative genes plays a key role in the development of most tumors. We report here the cloning of the mouse BTG3 gene and show that its human counterpart maps on chromosome 21. This evolutionarily conserved gene codes for a 30 kDa protein and is expressed in most adult murine and human tissues analyzed. However, we demonstrate that its expression is cell cycle dependent and peaks at the end of the G1 phase. This gene is homologous to the human BTG1, BTG2 and TOB genes which were demonstrated to act as inhibitors of cell proliferation. Its description allowed us to define better this seven gene family (the BTG gene family) at the structural level and to speculate about its physiological role in normal and tumoral cells. This family is mainly characterized by the presence of two conserved domains (BTG boxes A and B) of as yet undetermined function which are separated by a non-conserved 20-25 amino acid sequence.