The correct answer is "2 and 3 only" because stem cells can be derived not only from mammals but also from various organisms, including plants and certain invertebrates, making statement 1 incorrect. However, stem cells are indeed pivotal in drug screening as they can mimic human cells, allowing researchers to test the efficacy and toxicity of new drugs. Additionally, their ability to differentiate into various cell types positions them as a promising tool for medical therapies, such as regenerative medicine and tissue repair, validating statements 2 and 3.