Given the task definition and input, reply with output. In this task, you are given a sentence. You are expected to recognize the name of gene or protein. Although there might be several correct answers, you need to write one of them.

Interestingly , these " activation - defective " mutants segregated into two classes : 1 ) those that were unable to form dimers but that could still form higher order oligomers and transform cells , and 2 ) those that were defective for PDGF - R binding and were transformation - incompetent .
PDGF - R