Patent ID: 7250251

Claim:
A method for detecting fusion of an enveloped retrovirus to a target cell, the method comprising: contacting a target cell with an enveloped retroviral virion, the virion containing a chimeric viral protein comprising a reporter polypeptide operably joined to a viral accessory protein, wherein the reporter polypeptide provides a detectable signal by cleaving a substrate in the target cell upon intracellular delivery of the chimeric viral protein into the target cell cytoplasm, wherein the substrate is retained in the cytoplasm and is not present at a significantly detectable level in an endosome; and wherein the detectable signal is not detectable prior to said intracellular delivery into the target cell cytoplasm; and detecting the presence or absence of the detectable signal; wherein the presence of the detectable signal indicates the virion has entered the target cell by viral fusion and not by endocytosis.