We have already demonstrated that high quality SiNW-FETs can be used to sense very low concentration range (fM) of pathogenic factors in controlled PBS. In this work we show, for the first time, Silicon Nanowire Field Effect Transistors (SiNW-FET) as biosensors for the detection of antigen in breast tumor extract. We achieved the detection of exogenously added rabbit antigen in a much more complex environment, i.e. a human breast tumor extract. Our preliminary results show specific antigen detection with SiNW-FETs in the range of 5-200 fM. Further and most importantly, the wires sense the antigen in the presence of a 100 000 mass excess of non-specific protein, indicating that the sensor is extremely resistant to noise.