Increased HR activity in MM cells. HR activity was measured with the recombination plasmid substrate. Generation of the neomycin resistance as a measure of recombination activity was evaluated by counting neomycin-resistant colonies and total plasmid-transfected was evaluated by ampicillin-resistant colonies. The recombination frequency was calculated from the ratio of these 2 counts. Recombination frequency is presented as number of recombination events per million transfected plasmid copies.