RNA interference (RNAi) or Post-Transcriptional Gene Silencing (PTGS) is a conserved biological response to double-stranded RNA that mediates resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes.

Statement 1 is correct. RNAi is a powerful tool for developing gene-silencing therapies. It allows researchers to target specific genes and reduce their expression, which can be beneficial in treating various diseases.

Statement 2 is also correct. RNAi is being actively explored for cancer therapy. By targeting genes involved in cancer cell growth and proliferation, RNAi has the potential to be a novel approach for cancer treatment.

Statement 3 is incorrect. RNAi is not typically used in developing hormone replacement therapies. Hormone replacement therapies introduce hormones into the body, while RNAi focuses on silencing gene expression within cells.

Statement 4 is correct. RNAi can be used to develop crop plants resistant to viral pathogens. By introducing specific RNA molecules that target the virus, plants can be made more resistant to viral infections, improving agricultural yields.