Free left arms as precursor molecules in the evolution of Alu sequences

The dimeric Alu molecule of human and other primates is composed of a left and a right arm that are very similar but show characteristic differences. If the Alu sequence has arisen through the fusion of monomeric precursor molecules, the traces of such precursor genes are expected still to be present in contemporary primate DNA. We report finding seven independent human DNA sequences that qualify as descendants of a left-arm precursor gene. Some characteristics in primary and secondary structures of these sequences are described.