Loopback extensions on the ivt-MRP 3′ end are observed at low abundance by deep sequencing. a Approximately one per cent of observed 3′ ends contain complex sequences that are not explained by the in vitro template (violet slice). The majority of the reads from in vitro transcribed MRP (ivt-MRP) are the exact target sequence (grey slice), while the remainder can be categorized as single nucleotide additions to this designed end (+A in blue; +C in green; +G in yellow; +U in red), or other simple sequences (i.e. …CUCC, …CUGC, white slice). b Examples of possible duplex registers to template the observed sequences of loopback extensions.