This document discusses prognostic markers and their use in targeted cancer therapy. It defines prognostic markers as biological factors that can predict a patient's response to treatment. There are two main types - clinical markers like age and health status, and biological markers that can be measured in blood or tumor tissue, like gene or protein expression. Prognostic markers are used to select patients most likely to benefit from a targeted therapy, monitor treatment response, and predict cancer recurrence or progression. Examples given are BRAF and EGFR mutation status, and HER2 overexpression.