The support in question is not in MBS, but in treasuries.  But seeing as how spreads are seen to be about as tight as they're going to get, we wouldn't want to count on further tightening in order to believe MBS will not be facing similar battles in the near future.

Note on the chart how the 10yr had been having it's longest battle (successfully until just now!) with support levels at 3.70.  With that breakout, the risk of a reprice for the worse becomes a reality to a much greater extent than it has been.  Please be careful however, this is SUCH and early warning sign and we're getting it to you so quickly that it's far too soon to be sure that the bearish trend will continue.  Still...  If you were waiting for the very first potential sign of reprice risk increasing, now you have it.  Stay tuned.