Looking at the Trendline on an S&P 500 ETF, SPY, one can see that 135 is setting up as support on the whole rally since last October. If the market breaks below 135, I would not be surprised if SPY trades down towards its 200 day moving average - which is currently around 128. 135 would be about a 5% retrenchment from the 4 year highs of 142 in the market while 128 would be about a 10% retrenchment.
(Disclosure: Bellaire Capital Management, LLC currently holds SPY in client accounts. This is neither an offer to buy or sell any security and this blog post is for informational purposes only).