When tumor cells migrate from the primary (or original) cancer site to a distant region, metastatic cancer results. Some of the traits and properties of the originating tumor are still present in cancer that has metastasized (or spread) to an organ in a secondary location. For instance, breast cancer metastatic lesions in the brain will therefore resemble breast cancer rather than brain cancer. Cancer can spread metastatically almost anyplace in the body, and nearly all primary cancers can do the same. Common metastatic sites include the brain, bone, liver, and lung. Developing effective treatments for patients with cancers metastasizing to the brain remains a challenge due in part to difficulty getting drugs past the blood-brain barrier.