Fig. 6

A two-step model for a SIRP (suppressor of initiation but a requisite for progression) type of tumor suppressor to contribute to tumorigenesis. On one hand, a defect of SIRP tumor suppressor triggers genomic instability and tumor initiation. On the other, the same defect impairs cell proliferation and thus hinders further progression of the tumors. However, restoration of the SIRP activity in a later stage, or a secondary genetic change that compensates the SIRP defects, may confer synthetic viability to allow the tumor to overcome the growth-retardation barrier