1031 Background: Resistence mechanisms to T are still undefined. EGFR, pMAPK, pAKT and PTEN status by IHC were not correlated with response to T in our previously series of 45 HER2+ MBC pts. No data are reported about incidence of HER2, EGFR and PIK3CA genes mutations and their correlations with response to T. in HER2+ MBC pts. Methods: From 4/1999 to 3/2006, 133 consecutive pts were treated with T. Tumor tissues for this analysis were available from 41 pts. Genomic DNA was isolated from paraffin-embedded tumor specimens, amplified for HER2 (exons 19, 20, 21 and 22, encoding the kinase domain), EGFR (exons 18, 19, 20 and 21) and PIK3CA (exons 9 and 20, encoding a part of elical and kinase domains, respectively) genes by nested polymerase chain reaction and sequenced in both sense and antisense directions. Results: We found mutations of HER2 in 3 pts (7.3%), of EGFR in 6 pts (14.6 %) and PIK3CA in 5 pts (12.2%) with HER2+ tumors (Table); correlations with response to T are reported (Table). The same HER2 m...