This is a guest blog post from Eli Ceryak with Cushman & Wakefield.

San Francisco had another banner year in 2015. Office rents pushed up to an all-time high of $70.31 per square foot per year (Class A) and vacancy fell to 5.9%.

With 2016 well underway, there’s a free-flowing debate on where this market is headed. Will the market continue its long expansion? Is a day of reckoning coming? There are mixed signals, and here are 3 things we’ll be watching to inform our decision-making:

Sublease space: We think sublease space could be the leading indicator of a turning market. Several prominent companies have put large blocks of sublease on the market recently, including Salesforce, Twitter, Charles Schwab and Dropbox. However, most sublease space has leased quickly after hitting the market. Dropbox put 200,000sf on the market in advance of their relocation to a new headquarters, and quickly struck deals with Lyft for 100,000sf and Stripe for another 100,000sf. Salesforce also quickly found subtenants in a similar scenario last year. Sublease space represented 16.3% of the total vacancy at the end of the year (i.e. roughly 1/6 of the available space was sublease space, not direct space from landlords), which is higher than average but lower than the previous two quarters.

In short, demand continues to outstrip supply, and thus far the sublease space has served as a much needed relief valve. However, if companies like Salesforce, LinkedIn or Twitter shed significant space, that could swing the pendulum away from landlords.

New Construction: Office development in San Francisco has literally been a field of dreams: build it and tenants will come, likely with a heavy dose of pre-leasing . There is 2.4 million square feet of new construction slated for completion this year, which will increase the total supply in San Francisco by over 3% to 78 million square feet. Two million of that 2.4 million square feet has already been leased. Large prelease deals include LinkedIn‘s lease for 452,000sf at 222 Second; Salesforce’s 440,000sf lease at 350 Mission; and Dropbox’s lease for 300,000sf at 333 & 345 Brannan. 181 Fremont is the only building slated for completion in 2016 that significantly affects availability, with 413,000sf available at the end of the year. Starting in 2017, there are bigger blocks: Salesforce Tower (700,000sf available, roughly half the building), 350 Bush (370,000sf, and a rare north financial district development), the Exchange in Mission Bay (680,000sf) and Park Tower (750,000sf, slated for completion in early 2018). The pace of pre-leasing in these 5 buildings will be a significant barometer of where the market is headed and could heavily impact rents (positively or negatively) as a result.

Valuations & Exits: Many of San Francisco’s bellwether public companies have shed significant value over the past year. Salesforce is a notable exception, and standout, among major San Francisco tech companies with a 16.77% stock price appreciation over the past year and a $6.53 billion increase in market capitalization during that time. However, Twitter’s stock price has decreased by 55% in the last year leading to a $14.9 billion decrease in market capitalization. Other companies hit by declining shares are Fitbit (44% stock decline, $2.7 billion decrease in market value since their June 2015 IPO), Yelp (62.3% price decline in the past 12 months, $2.7 billion in lost market value), LinkedIn (11.6% price decline, $3.54 billion decrease), Lending Club (62.5% stock decrease, $4.8 billion decrease in market value) and Square (33% stock decline, $1.39 billion decrease in market value since their November IPO).

The challenge with the declining values is twofold. First, it could lead to those companies shedding jobs and real estate as they try to streamline operations and keep shareholders happy. Second, it could freeze the exit opportunities for the Uber’s, Airbnb’s, Pinterest’s, Stripe’s and Dropbox’s of the world, and the hundreds of other local start-ups that have similar aspirations. The Wall Street Journal just highlighted the fact that in January there were no I.P.O’s in the U.S., the first month that had happened since September 2011. We’ll be watching the public markets, private investment and M&A activity closely to see how it all plays out.