PDGFRA and gastrointestinal stromal tumor: Sirolimus, another mTOR inhibitor, in association with TKIs (PKC412 or imatinib) showed an antitumor activity in three GIST patients harbouring exon 18 PDGFRA-D842V mutation, that is well known to confer resistance to imatinib in vitro and in vivo [15,16].