That just means that they could only detect a single gene in each of the two patients. (one was the gag gene and the other was the integrase gene.) That just means that the copy numbers of the viruses were very low. That means that the detection of sequences was random, or unreliable, in that they could only detect a single gene in each sample. If the virus was present, then it might be expected that more genes could be detected, but if copy numbers are at the limits of detection, then maybe it's not so surprising. This suggests that a whole virus is not present in the samples, but just a fragment of viral DNA. I originally thought that 'evolutionary relic retrovirus sequence' is a reference to human endogenous retroviruses (HERVs), but now I think they are talking about any endogenous retroviruses, including mouse ERVs. (If they are only talking about HERVs, then the paragraph doesn't seem to make much sense, as they just start talking about HERVs out of nowhere.) If they are talking about all endogenous retroviruses, then they just mean that more research is needed before any conclusions can be made in this field of research. They are raising a question about the nature of the sequences, and where they originate from. Edit: I've completely changed the last section of my text.