RNA interference (RNAi) technology has gained popularity in the last few years. Why? 1. It is used in developing gene silencing therapies. 2. It can be used in developing therapies for the treatment of cancer. 3. It can be used to develop hormone replacement therapies. 4. It can be used to produce crop plants that are resistant to viral pathogens. Select the correct answer using the code given below.

The correct answer is A, 1, 2 and 4. RNA interference switches particular genes off, and every use in statements 1, 2 and 4 depends on switching a gene off. In RNAi, short double-stranded RNA molecules, such as small interfering RNA or microRNA, guide a protein complex to the matching messenger RNA and destroy it, so the protein coded by that gene is never made. Andrew Fire and Craig Mello won the 2006 Nobel Prize in Medicine for discovering it. Doctors use it to build gene silencing drugs for inherited disorders, researchers use it to silence the genes that drive tumour growth, and plant breeders use it to give crops resistance by silencing the genes of an invading virus. Statement 3 is wrong because hormone replacement means supplying a missing hormone, which needs a gene to be expressed, not silenced. Option B is wrong because it includes statement 3 and drops statements 1 and 4. Option C is wrong because it includes statement 3. Option D is wrong because it leaves out statement 2. Exam tip: RNAi turns genes off; it cannot add a missing hormone.