The present invention provides methods for the determination of the activation level of Receptor Tyrosine kinases, e.g. phosporylated HER3, for the selection of patients for disease treatment. Methods are also provided for the evaluation of the biological and pharmacodynamic effects of an active substance and/or its efficacy in disease treatment, utilizing a tissue sample from a test subject, for example tumor material or normal tissue such as skin or hair follicle. Further, methods for the treatment of HER receptor-associated diseases are disclosed.