Updated project metadata. This study represents a proteomic resource for HNSCC and SQCLC with quantitative protein expression data for 7800 proteins and provides a proteomic diagnostic signature for classification for undetermined secondary lung tumors in HNSCC patients. By quantitative mass-spectrometry-based proteomics, we characterized a cohort of 63 squamous cell carcinomas of the lung (SQCLC), 49 of the head and neck region (HNSCC) and 51 HNSCC-correlated lung tumors with squamous cell histology that evolved in the course of the disease.