That has not, however, discouraged investors from making more bearish bets as the bull grows older and older. Goldman Sachs Group Inc. strategist David Kostin wrote in arecent report that clients are on the prowl for short ideas. And Markit's average cost-to-borrow score has risen to 1.36 for Russell 3000 Index companies, from 1.27 at this time last year and 1.26 in 2013, according to data compiled by Bloomberg. There are 17 companies with the highest-possible score of 10, compared with eight last year.

Among those 17 most-expensive stocks to short, the biggest gain in April has been by Molycorp Inc., which has more than doubled this month but has been a short's dream-come-true over the long term with a drop of 99 percent from a 2011 peak. Of the other 17 stocks causing short-sellers pain this month, with gains of more than 20 percent so far, are Shake Shack Inc., ARC Group Worldwide Inc., Organovo Holdings Inc., Approach Resources Inc. and Northwest Biotherapeutics Inc.

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