Adeno-associated virus (AAV) is a small, non-enveloped single-stranded DNA virus. Upon infection to humans, they cause only a very mild immune response, and are basically thought to be non-pathogenic. AAV-derived vector (AAV vector) can infect both dividing and non-dividing cells, and the viral genome stays in an extrachromosome without integrating into the genome of the host cell. Because of these features, AAV vector is one of the most promising gene delivery agent for gene therapy. AAV has a very small genome, which is consisted of two genes encoding a replicase and capsids, respectively. Of these two genes, the structural capsid gene defines the infectious characteristics of the AAV. Recently, many useful capsid variants have been developed. In particular, AAV-PHP.B, a blood-brain-barrier (BBB)-penetrating AAV capsid variant, has had significant impacts on neuroscience. After intravenous infusion, AAV-PHP.B efficiently penetrates the mature mouse BBB, and transduces the whole brain. Meanwhile, we have developed a variety of brain cell type-specific promoters. A combination of BBB-penetrating AAV-PHP.B with the cell type-specific promoter allows us to express a gene of interest in a specific cell population. In this presentation, I will show several application examples of cell type-specific transgene expression by AAV-PHP.B with a cell type-specific promoter. I also present our recently developed method of cell type-specific conditional gene knockout in flox mice using BBB-penetrating AAV vectors.