KRAS and neoplasm: The selection of the tissue blocks for analysis is of clinical relevance for the following reasons: i) The tissue sampled for genotyping may contain a small population of malignant cells and a large population of stromal and inflammatory cells; ii) the potential genetic heterogeneity of the tumor tissues in terms of KRAS and/or BRAF mutational status; and iii) the genetic heterogeneity of multiple types of colorectal cancer may result from one or both of the above-mentioned circumstances or from an independent tumor origin.