Structure and variability of recently inserted Alu family members.

The HS subfamily of Alu sequences is comprised of a group of nearly identical members. Individual subfamily members share 97.7% nucleotide identity with each other and 98.9% nucleotide identity with the HS consensus sequence. Individual subfamily members are on the average 2.8 million years old, and were probably derived from a single source 'master' gene sometime after the human/great ape divergence. The recent Alu family member insertions provide a better image of the structure of Alu retroposons before they have had the opportunity to change significantly. All of the HS subfamily members are flanked by perfect direct repeats as a result of insertion at staggered nicks. The 'master' gene from which the HS subfamily members were derived had an oligo-dA rich tail at least 40 bases long. The 'master' gene is very rich in CpG dinucleotides, but nucleotide substitutions within subfamily members accumulated in a random manner typical for Alu sequence with CpG substitutions occurring 9.2 fold faster than non-CpG substitutions.