Hexameric helicases are thought to act by encircling only one strand of DNA upon which they track and exclude the non-tracking strand to the outside of the ring, thereby acting as a wedge to split DNA in a process often referred to as steric exclusion and illustrated in Figure 1A (Bell and Labib, 2016; Enemark and Joshua-Tor, 2008; Lyubimov et al., 2011). Whether a helicase functions by steric exclusion is determined by biochemical experiments that place a bulky block on one or the other strand of the duplex. For a steric exclusion helicase, a block placed on the non-tracking strand (i.e. the strand that is excluded from the central channel) does not inhibit helicase unwinding, while a block placed on the tracking strand stops the helicase because the bulky block cannot fit through the central channel.