 Because AAV can deliver transgenic material in a non-replicating form, it is a strong candidate for gene therapy and is currently used in about 8% of clinical trials.Hepatitis D virus (HDV) is an example of a replication defective, helper dependent ssRNA virus because it requires Hepatitis B virus (HBV) to provide HBV surface antigen (HBsAg) for the encapsidation of its genome.The envelope proteins on the outer surface of HDV are entirely provided by HBV.