Sparse labeling of neurons contributes to uncovering their morphology,and rapid expression of a fluorescent protein reduces the experiment range.To achieve the goal of rapid and sparse labeling of neurons in vivo,we established a rapid method for depicting the fine structure of neurons at 24 h post-infection based on a mutant viruslike particle of Semliki Forest virus.Approximately 0.014 fluorescent focus-forming units of the mutant virus-like particle transferred enhanced green fluorescent protein into neurons in vivo,and its affinity for neurons in vivo was stronger than for neurons in vitro and BHK21 (baby hamster kidney) cells.Collectively,the mutant virus-like particle provides a robust and convenient way to reveal the fine structure of neurons and is expected to be a helper virus for combining with other tools to determine their connectivity.Our work adds a new tool to the approaches for rapid and sparse labeling of neurons in vivo.