Unlike other restriction enzymes, Type I R-M enzymes cut their target DNA at random sites and the site of cleavage can be up to 20 kbp from the target recognition sequence.  This distal cleavage is accomplished through a process of DNA translocation, in which the target DNA is moved through the enzyme complex until cleavage occurs (Figure 2).  Studier & Bandyopadhyay (1988) proposed a model for this cleavage that depended upon collision between two translocating enzymes for DNA cleavage (Figure 3).  This model was revised slightly by Szczelkun et al. (1996, 1997) who suggested that anything that is capable of blocking translocation (including lack of DNA on circular molecules) will produce cleavage.  However, very few DNA-binding molecules are able to block translocation and only Holliday structures, or another translocating Type I R-M enzyme, have this effect (Janscak et al., 1999).