Background: Abnormalities in EGFR signaling targets are associated with C resistance but no biomarker of C resistance has been identified in HNSCC. We hypothesized that cases with loss of PTEN protein expression (PTEN null) or PIK3CA mutation would display C resistance in HNSCC. Methods: E5397 was a phase III trial of CDDP plus P or CDDP plus C and enrolled 117 eligible and evaluable pts. PIK3CA and PTEN were analyzed for 52 and 67 consented pts, respectively. PTEN expression (PTEN Cell Signaling Technology, Cat. 9559) was determined by automated quantitative analysis (AQUA) on the PM-2000 (HistoRx, New Haven) using a cutpoint generated in 5 HNSCC tissue microarrays, each consisting of HNSCC as well as positive (small intestine, median AQUA score 2833.2) and negative controls (breast and colon carcinoma, median AQUA score 205.5). A cutpoint of 570 provides 100% specificity, 100% sensitivity, and identified 30% of the HNSCCs as PTEN null, consonant with the literature. The 3 most common PIK3CA mutations (E542K and E545K in exon 9 and H1047R in exon 20) were determined by BEAMing (Inostics, Heidelberg, Germany). Response, overall survival (OS) and progression-free survival (PFS) were compared between PTEN null or PIK3CA mutated pts and all others. Log rank and multivariable Cox proportional hazards modeling were used to calculate p values. Results: 23/67 (34%) tumors were PTEN null and 2/52 (4%) had PIK3CA mutations (E542K and E545K). Both tumors with PIK3CA mutation had PTEN expression. No statistically significant differences in response, OS or PFS were noted in this small sample. However, among PTEN expressing/PIK3CA WT pts, median PFS increased to 4.2 months (m) for C (N=22) from 2.9 m for P (N=26) (Wald p=0.07), compared with 4.6 m for C (N=12) and 3.5 m for P (n=13) among the PTEN null/PIK3CA mutated (Wald p=0.60). Conclusions: The PTEN loss or PIK3CA mutation signature warrants further investigation as a predictor of C resistance.