The term originally referred to the southern part of the San Francisco Bay Area in Northern California, United States. It encompasses the northern part of the Santa Clara Valley and adjacent communities. According to the Joint Venture Silicon Valley Network, Silicon Valley spreads over the whole of Santa Clara County, Foster City, San Mateo, Belmont, San Carlos, Redwood City, Atherton, East Palo Alto, Woodside, Portola Valley, Menlo Park in San Mateo County, Union City, Fremont and Newark in Alameda County, Scotts Valley in Santa Cruz County. The U.S. Highway 101 runs north-south through the region.