The correct answer is 2 and 3 only.

Explanation:

1. Stem cells can be derived from mammals only. - This statement is incorrect as stem cells can be derived from various sources such as embryonic tissues, bone marrow, adipose tissue, and cord blood in humans and other animals.

2. Stem cells can be used for screening new drugs. - This statement is correct. Stem cells are useful in drug screening because they can differentiate into various types of cells, making them a valuable tool for testing the effectiveness and toxicity of new drugs.

3. Stem cells can be used for medical therapies. - This statement is correct. Stem cells have the potential to regenerate damaged or diseased tissues and organs, making them a promising avenue for medical therapies in the future. For example, stem cells can be used to treat certain types of cancer, blood disorders, and degenerative diseases such as Parkinson`s and Alzheimer`s.