GDX is one of the most popular funds in the global gold-mining segment, a trading powerhouse with deep primary, secondary and derivatives markets. Following a 2013 overhaul of the fund`s index, GDX is no longer limited to US-listed firms, nor does it filter out firms that have hedged their gold exposure with derivatives. In addition, it includes firms mining other precious metals in addition to gold, so it`s not entirely pure-play gold miners. GDX includes all the major names, but the fund`s broader take on the gold-mining space means that the big firms get less weight than in our neutral benchmark. As an industry benchmark, GDX falls a bit short, but it's still hugely popular.