The pi calculus has been applied to modelling biochemical networks. In these applications, the modelling is done without considerations to exceptions. The Ipi calculus, the Interference pi calculus, is introduced to describe the signal transduction with aberrance. The calculus is obtained by adding two aberrant actions into the pi calculus and a tag system to check existing aberrance. A model of the signal transduction, RTK-MAPK, with the aberrant Ras is highlighted to illustrate the expressive power of the Ipi calculus.