We have determined sequences of PCR-amplified B1 elements from hamster and rat 
(Myomorpha), chipmunk (Sciuromorpha), and guinea pig (Caviomorpha). Between 
three and six B1 subfamilies were found in these species. In the phylogenetic 
analysis B1 sequences of hamster, mouse, and rat clustered separately from those 
of chipmunk and those of guinea pig. This is consistent with an independent 
evolution of B1 elements in separate rodent lineages. We exclude the possibility 
of convergent mutations to explain certain diagnostic characters within the 
modern B1 quasi-dimers and view these elements as mosaic structures assembling 
preexisting mutations. Furthermore, the presence of Alu-like structural motifs 
supports the hypothesis of the monophyletic origin of Alu and B1 repeats, i.e., 
from a common 7SL RNA-derived retroposing monomeric element.