Expression difference map of gastric fluid and pellet extract proteins of training set samples on four ProteinChip arrays, displayed in two-way hierarchical clustering. Significant proteomic features are displayed vertically. The intensity of the grayscale indicates the degree of relative protein level, higher or lower than the median value. Patient cases are presented horizontally; most gastric cancer patients are tightly clustered together. This figure shows the upper quartile of the full image (please see Additional file 2 for the full image).