RNA molecules that can be introduced into cells through transfection include:

1. Messenger RNA (mRNA): mRNA carries the genetic information from DNA to the ribosome, the protein synthesis factory of the cell, where it serves as a template for protein synthesis. mRNA transfection is used to induce cells to produce a specific protein. It is used in basic research to study protein function and in clinical applications, such as mRNA vaccines (e.g., Pfizer-BioNTech and Moderna COVID-19 vaccines).
2. Small interfering RNA (siRNA): siRNAs are small, double-stranded RNA molecules that can bind to specific messenger RNAs and trigger their degradation, effectively silencing the expression of the corresponding gene. siRNA transfection is often used in research to knock down specific genes and study their function. It also has potential therapeutic applications in diseases caused by overexpression of certain genes.
3. MicroRNA (miRNA): miRNAs are small, single-stranded RNA molecules that can bind to messenger RNAs and inhibit their translation into protein. Unlike siRNAs, which usually bind perfectly to their target mRNAs and lead to their degradation, miRNAs often bind imperfectly and mainly inhibit translation. miRNA transfection is used to study gene regulation and has potential therapeutic applications in diseases caused by dysregulation of gene expression.
4. Short hairpin RNA (shRNA): shRNA is a small, hairpin-shaped RNA molecule that can be processed within the cell to form siRNA. shRNA is often delivered into cells using viral vectors, which can integrate into the cell’s genome and produce a continuous supply of siRNA, leading to long-term gene silencing.
5. Long non-coding RNA (lncRNA): lncRNAs are a large and diverse class of transcribed RNA molecules with a length of more than 200 nucleotides that do not encode proteins. Some lncRNAs have been found to have various roles in regulating gene expression, cell development, and differentiation. Transfection with lncRNAs is used in research to study these processes.
6. Circular RNA (circRNA): circRNAs are a type of RNA which, unlike linear RNA, forms a covalently closed continuous loop. Some circRNAs have been found to act as sponges for miRNA, thereby preventing the miRNA from inhibiting its target mRNA. Transfecting cells with circRNAs can be used to study these interactions and their effects on gene expression.

Each type of RNA molecule can be used for different purposes in RNA transfection, and the choice depends on the goals of the experiment or treatment.