The question of the origin of the B1 family of rodents is addressed. The modern 
B1 elements are similar to the left Alu monomer, but with a 9 bp deletion and a 
29 bp duplication. Search of databases for B1 elements that do not exhibit those 
modern features revealed sequence fragments that are very similar to the free 
left Alu monomers (FLAMs) described in the primate genomes. In addition, the 
analysis reveals elements that have 10 bp or 7 bp deletion in place of the 9 bp 
deletion but without the 29 bp tandem duplication. The elements described define 
families of proto B1 elements (referred as PB1, PB1D10 and PB1D7) that appeared 
before the first modern B1 element. A phylogenetic reconstruction suggest that 
the origin of Alu and B1 families took place before the divergence between the 
primate and the rodent lineages and that each family has followed different 
evolutionary routes since this radiation.