Classification of 2382 potentially active L1 elements residing in the mouse genome sequence (NCBI m35). A. Distribution of L1s among subfamilies. A, TF and GF correspond to active mouse L1 subfamilies. The small number of L1s that appear related to the inactivated F subfamily are marked with F and those lacking monomers are marked with N/A. B. Distributions of the lengths of the internal promoter regions among the three active families. The longest promoter discovered is composed of 28 monomers (here included in the 14+ class) and is a feature of a potentially active L1 element belonging to the A subfamily located on chromosome 2 (80663469-80649072).