We demonstrate that single walled carbon nanotube field effect transistors (swCN-FET) decorated with single stranded DNA (ss-DNA) form a new, versatile, and highly sensitive class of nanoscale chemical sensors. These sensors are based on ss-DNA as the chemical recognition site and swCN-FET as the electronic read-out component. The ss-DNA decorated swCN-FETs are sensitive to chemical species (odors) that do not cause a detectable response in bare, as-fabricated swCN-FETs. Furthermore, odor responses ss-DNA decorated FETs differ in sign and magnitude for different odors, and the response characteristics depend on the base sequence of the ss-DNA used to decorate the swCN. The sensor surface is self-regenerating: samples maintain a constant response with no need for sensor refreshing through at least several dozens gas exposure cycles. These remarkable attributes suggest that DNA-decorated swCN-FET sensors could be created with sensitivity to a large variety of compounds, as required for "electronic-nose" and "electronic tongue" applications in medicine and homeland security.