As we have seen, Hox genes and the proteins they encode are very similar in a wide range of organisms. As a result, they can be identified by their similarity in DNA or amino acid sequence. Molecules that are similar in sequence among distantly related organisms are said to be evolutionarily conserved. Their similarity in sequence suggests that they were present in the most recent common ancestor and have changed very little over time because they carry out a vital function. Many transcription factors important in development are evolutionarily conserved. An impressive example is found in the development of animal eyes.