Whole-section analysis with ISH or immunohistochemistry facilitates characterization of expression pattern heterogeneity within a tumor, a feature that can be missed with RNA sequencing analysis of random biopsies, as exemplified by our experience with MUC5AC and APOBEC1. When used with reliable antibodies, immunohistochemistry can also identify tumor-specific changes that can escape detection by ISH. This evidence concerns the gene APOBEC1 and neoplasm.