Background: We identified HER2 amplification as a potential onco-driver and marker of de novo resistance to anti-EGFR therapy in mCRC PTS for which other known genetic alterations conferring resistance to anti EGFR antibodies were excluded. Exploiting direct transfer xenografts of mCRC surgical samples in mice (xenoPTS), we conducted a multi-arm study in HER2-amplified xenoPTS showing that combinations of lapatinib (L) and trastuzumab (T), or L and pertuzumab (P) induced long-lasting tumor regressions while monotherapy with L led to stabilization and either monotherapy with T or P were ineffective (Bertotti et al. Cancer Discov 2011). The combination of P+T has a strong rationale for treatment of HER2-amplified mCRC, since combining the two agents is synergic in HER2+ breast cancers failing T (Baselga et al. JCO 2010), suggesting a cooperative mechanism of inhibition. On these findings we designed the HERACLES trial. Methods: HERACLES is an independent Phase II, 2-sequential cohort trial, assessing the response rate (RR) of T combined with either L (Cohort A) or P (Cohort B), in m CRC PTS harbouring an amplified HER2 tumor (SISH). Endpoints are RR and PFS. For each cohort sample size was calculated according to the Fleming & Hern 1- stage design under identical assumptions: H0 = RR 10%, H1= RR 30%. With a =0.05, power=0.85, 27 patient are required in each cohort (54 patients overall). A or B will be considered positive if ≥ 6 responses/27 PTS are observed. Detection and quantitation of genetic alterations in circulating tumor DNA (liquid biopsy) and Her2 ECD in plasma as potential markers of secondary resistance will be done q14 days until relapse. Eligibility: CRC histology with KRAS WT, HER2 IHC 3+ ≥50% cells, prior treatment with fluoropirimidines, oxaliplatin, irinotecan, anti EGFR moABs based regimens ± Bevacizumab; measurable disease (RECIST v1.0), ECOG PS ≤1, adequate organ function. Response is assessed q8w. Treatment: L 1000 mg daily po + T 4 mg/kg iv load, then 2 mg/kg iv weekly. Enrollment: since trial start (8/12/12) 198 PTS have been HER2 screened, 10 found positive and 7 are in treatment. EudraCT number : 2012-002128-33. Clinical trial information: 2012-002128-33.