Unlike human Ll retrotransposons, the 5 ' UTR of mouse Ll elements contains
tandem repeats of similar to 200 bp in length called monomers. Multiple Ll
subfamilies exist in the mouse which are distinguished by their monomer se
quences. We previously described a young subfamily, called the T-F subfamil
y, which contains similar to 1800 active elements among its 3000 full-lengt
h members. Here we characterize a novel subfamily of mouse LI elements, G(F
), which has unique monomer sequence and unusual patterns of monomer organi
zation. A majority of these G(F) elements also have a unique length polymor
phism in ORF1. Polymorphism analysis of G(F) elements in various mouse subs
pecies and laboratory strains revealed that, like T-F, the G(F) subfamily i
s young and expanding. About 1500 full-length G(F) elements exist in the di
ploid mouse genome and, based on the results of a cell culture assay, simil
ar to 400 G(F) elements are potentially capable of retrotransposition. We a
lso tested 14 A-type subfamily elements in the assay and estimate that abou
t 900 active A elements may be present in the mouse genome. Thus, it is now
known that there are three large active subfamilies of mouse Lis; T-F, A,
and G(F), and that in total similar to 3000 full-length elements are potent
ially capable of active retrotransposition. This number is in great excess
to the number of Ll elements thought to be active in the human genome.