I could not find any mention of DERSE in a Google search. But, from my experiences in the field, I believe that DERSE stands for Detection of Exogenous Retroviral Sequence Elements. Each retrovirus has some sites that have a sequence shared with other retroviruses; they also have sites that are unique. I believe from the context that DERSE is a method of detecting these unique sequences. Hopefully it can do this consistently, without losing sequences from some variants of the viral genome, as PCR can do if there is a mutation in the primer region.