In recent years, UCSC added browser support for four compressed binary indexed data formats: BigBed and BigWig (Kent et al., 2010), both developed at UCSC, Binary Alignment/Map (BAM) (Li et al., 2009) and Variant Call Format (VCF)/tabix (Danacek et al., 2011). This allowed individuals to quickly and efficiently view and share genome-wide data hosted on their own local servers using the browser’s well-established custom track mechanism. However, the limited configuration and organization options imposed by custom tracks presented a barrier to full integration of large datasets into the browser, leading many research groups to set up mirrors to visualize their tracks in a full local instance of the browser. Mirrors pose their own drawbacks: they tend to have limited visibility and distribution within the research community and incur a local maintenance overhead.
