This article presents a detailed protocol for using small interfering RNAs (siRNAs) to knock down gene expression in mammalian somatic cells. The study describes the use of siRNAs, which are 21- to 23-nt double-stranded RNA molecules that trigger sequence-specific mRNA degradation. The method is highly efficient and does not activate the unspecific interferon response, making it suitable for studying gene function in cultured mammalian cells. The authors provide a comprehensive guide on selecting siRNA sequences, preparing siRNA duplexes, and performing transfections. They also describe methods for detecting gene knockdown, including immunofluorescence, Western blotting, and luciferase assays. The protocols are designed to be applicable to a wide range of mammalian cell lines and can be used for both basic research and therapeutic applications. The study highlights the potential of siRNAs as a powerful tool for functional genomics and gene therapy.This article presents a detailed protocol for using small interfering RNAs (siRNAs) to knock down gene expression in mammalian somatic cells. The study describes the use of siRNAs, which are 21- to 23-nt double-stranded RNA molecules that trigger sequence-specific mRNA degradation. The method is highly efficient and does not activate the unspecific interferon response, making it suitable for studying gene function in cultured mammalian cells. The authors provide a comprehensive guide on selecting siRNA sequences, preparing siRNA duplexes, and performing transfections. They also describe methods for detecting gene knockdown, including immunofluorescence, Western blotting, and luciferase assays. The protocols are designed to be applicable to a wide range of mammalian cell lines and can be used for both basic research and therapeutic applications. The study highlights the potential of siRNAs as a powerful tool for functional genomics and gene therapy.