On Thursday, the SPDR S&P Biotech ETF (NYSEArca: XBI) jumped 7.5% and touched a fresh all-time high on volume that was more than triple the daily average.

None of the other four biotech ETFs were up more than 1.3%. The reason for XBI’s good fortune on Thursday is easy to explain. Intercept Pharmaceuticals (NasdaqGM: ICPT) soared an astounding 281% after trials for the company’s liver disease treatment proved successful. XBI is the only one of the five biotech ETFs to have exposure to Intercept and although the ETF’s allocation to the stock heading into Thursday was just 1.61%, Intercept’s 281% gain translated to a pop of 7.5% for XBI. [Intercept’s Surge Boosted This ETF]

Obviously, it is not everyday that any one stock sparks a 7.5% in an equal-weight ETF, which XBI is. Nor is it everyday a biotech stock nearly quadruples in a single trading session, but if certain smaller biotechs fulfill their potential this year, XBI could see some more impressive performances thanks to stocks that only have small weights in the fund. To be clear, no stock accounts for more than 2.72% of XBI’s weight.

Start with Arena Pharmaceuticals (NasdaqGM: ARNA). Arena’s approved obesity drug Belviq has started to show promise in recent weeks and Jefferies has a $12 price target on the shares while Thomson/First Call sees $8, reports Lee Jackson for 24/7 Wall Street. Split the difference and call that $10 and Arena still offers significant upside from Thursday’s close just over $6. The stock is almost 1.5% of XBI’s weight.

Then there is Galena Biopharma (NasdaqGM: GALE), which 24/7 Wall Street said “may be the home run that biotech investors are looking for. The web site went on to note that Galena could be a takeover target and that volume has recently been increasing in the name. The latter point is certainly accurate as Galena’s turnover was more than triple the daily average Thursday. The stock is XBI’s third-largest holding at a weight of 2.11%.