You have to keep in mind that mutant EGFR we can look among the cancer patients is the result of the "Darwin-like evolutionary phenomenon." Needless to say, both of gain-of-functional and loss-of-functional mutations occur in EGFR, but given that EGFR activates cellular survival and proliferation, only the former becomes predominant. By striking contrast, the latter becomes disappear due to the cellular competition.  

 Mostly dominant.