A potential pitfall in using the mcrA gene for detection is the high degree of DNA sequence conservation between mcrA and mrtA, the equivalent gene in the MCRII complex, a complex found in a limited number of methanogen species. This means that detection based on primers for PCR designed to conserved regions in mcrA is also likely to detect mrtA sequences, complicating the analysis. Fortunately, the mrtA gene sequences form a coherent group close to the Methanococcales (Fig. 3) and therefore are readily distinguishable from mcrA gene sequences. This close proximity has been reported previously and led to the suggestion that the mrtA gene has arisen by lateral transfer from the Methanococcales rather than gene duplication (Reeve et al., 1997). There were, however, noticably more mrtA sequences than mcrA Methanobacteriales sequences, suggesting overrepresentation of this sequence type in the analysis. This is likely to be due to PCR bias, amplifying the mrtA sequences preferentially, rather than a wider distribution of mrtA genes in the methanogen community, as the mrtA gene sequences group closely with a single Methanobacteriales mrtA sequence from Methanobacterium formicicum.