The revival of the Mid-Market part of the downtown area in San Francisco after Twitter sets up headquarters here in April 2011. Twitter signed a lease till 2021 with Shorenstein Properties for a 11 story building built in 1937, for 295,000 square feet. Other developers are moving into San Francisco. Tishman Speyer is constructing two office buildings in the area called South of Market in San Francisco. Rents are now at $53.84 per square foot in San Francisco in the fourth quarter 2013 compared to $46.12 in the third quarter, according to C.B.R.E. Vacancy levels have dropped from 9.7% in 2012 to 8.2% in October 2013, and down from 15% in 2010.