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SUMMARY A recent article in Cell reported a new form of modified rabies virus that was apparently capable of labeling neurons “without adverse effects on neuronal physiology and circuit function”. These “self-inactivating” rabies (“SiR”) viruses differed from the widely-used first-generation deletion-mutant (ΔG) rabies viruses only by the addition of a destabilization domain to the viral nucleoprotein. However, we observed that the transsynaptic tracing results from that article were inconsistent with the logic described in it, and we hypothesized that the viruses used were actually mutants that had lost the intended modification to the nucleoprotein. We obtained samples of two SiR viruses from the authors and show here that, in both “SiR-CRE” and “SiR-FLPo”, the great majority of viral particles were indeed mutants that had lost the intended modification and were therefore just first-generation, ΔG rabies viruses. We also found that SiR-CRE killed 70% of infected neurons in vivo within two weeks. We have shown elsewhere that a ΔG rabies virus encoding Cre can leave a large percentage of labeled neurons alive; we presume that Ciabatti et al. found such remaining neurons at long survival times and mistakenly concluded that they had developed a nontoxic version of rabies virus. Here we have analyzed only the two samples that were sent to MIT by Ciabatti et al., and these may not be from the same batches that were used for their paper. However, 1) both of the two viruses that we analyzed had independently lost the intended modification, 2) the mutations in the two samples were genetically quite distinct from each other yet in both cases caused the same result: total or near-total loss of the C-terminal modification, and 3) the mutations that we found in these two virus samples perfectly explain the otherwise-paradoxical transsynaptic tracing results from Ciabatti et al.’s paper. We suggest that the SiR strategy, or any other such attempt to attenuate a virus by addition rather than deletion, is an inherently unstable approach that can easily be evaded by mutation, as it was in this case.