ISRm2011-2 was originally thought to be very similar (restriction site, length, copy number) to ISRm1 from Rhizobium meliloti 1021 (Labes and Simon, 1990). However, DNA sequencing clearly indicated its relationship to the IS630 family (Selbitschka et al., 1995). At least 8 ISRm2011-2 copies are present in the Rhizobium meliloti 2011 genome, including one copy on each megaplasmids I and II (Labes and Simon, 1990). Moreover, ISRm2011-2 was found in all the Rhizobium meliloti isolates tested, with an average copy number of 11. ISRm2011-2 was also shown to promote transcription of adjacent genes. ISRm2011-2 harbours two overlapping ORFs (94-501 and 435-1040), with a potential frameshifting site within the overlap. It is suggested that, like for the IS3 family, the ISRm2011-2 putative transposase corresponds to a transframe protein (94-435, 435-1040). This assumption remains however to be confirmed. Its is also interesting to note that ISRm2011-2 ORF1 displays more similarity with ORF1 of the IS3 family members (IS911 or IS1236) than with the N-terminal part of the IS630 family transposases ! (Selbitschka et al., 1995).