The sensitive analysis of proteins is central to disease diagnosis. The detection and investigation of angiogenic and inflammatory ligands in the tumor tissue can further improve the level of knowledge of the cancer disease by capturing the heterogeneity and the complexity of the tumor microenvironment. In previous works, we demonstrated that high-quality silicon nanowire field-effect transistors (SiNW-FETs) can be used to sense very low concentration (fM) of pathogenic factors in controlled phosphate buffered saline. In this paper, we show SiNW-FETs as biosensors for the detection of cancer markers in tumor extracts, as proof of our technology to successfully work on real patients' sample. In particular, we achieved the detection of exogenously added rabbit antigen in a much more complex environment, i.e., a human breast tumor extract. Our results show specific and high sensitive antigen detection with p-type SiNW-FETs in the femtomolar range. Further and most importantly, the wires sense rabbit antigen molecules in the presence of a 100 000 mass excess of nonspecific protein, indicating that the sensor is extremely resistant to noise.@en