question: Which is the main difference between Alu and B1 repeats?
answer: B1 is a murine homolog of the human SINE Alu. B1 (Alu-equivalent) is a murine short interspersed element whose amplification probably involved an RNA intermediate. The modern B1 elements are similar to the left Alu monomer, but with a 9 bp deletion and a 29 bp duplication.