$z\gtrsim1.5$ to be detectable with current instruments [@Chiang2013; @Muldrew2015]. A targeted approach reduces the strain on telescope time and can pinpoint clusters even at the highest redshifts. However, the right beacon needs to be used.

Radio-loud active galactic nuclei (RLAGN) preferentially reside in dense environments at high redshift [e.g. @Galametz2012; @Wylezalek2013], which are significantly denser than the environments of radio-quiet galaxies of the same stellar mass [@Hatch2014], which is also predicted from models [@Orsi2015]. These RLAGN provide one of the most efficient ways to find and study large-scale structure formation, particularly at the highest redshifts. However, if RLAGN preferentially reside in clusters of a certain age or stage of collapse, then our view of cluster formation will be biased. For example, @Simpson2002 and @vanBreukelen2009 suggest distant radio galaxies pinpoint merging clusters.

Most confirmed cluster progenitors, known as protoclusters, have been identified with Ly$\alpha$ emitters, H$\alpha$ emitters, or Lyman-break galaxies, which are tracers of young systems [e.g. @Overzier2005; @Venemans2007; @Cooke2014]. So the methods by which protoclusters have been identified preferentially pinpoint young, forming galaxies, and clusters that contain mature, passively evolving galaxies may be missed. By contrast, large cluster surveys using *Spitzer* data are not intrinsically biased to star-forming galaxies [@Simpson1999; 