San Francisco is a port city in California, USA. In terms of population, it's the fourth largest city in California. San Francisco is close to the world-famous high-tech industrial zone, Silicon Valley, and is the world's most important high-tech research and development base as well as the most important financial center in the western United States. Wuhan and San Francisco established friendly exchange city relations in 2013 and carried out in-depth "twin valleys" cooperation between Wuhan Optics Valley and San Francisco Silicon Valley.