# If you received a college degree and/or completed any of the prerequisite science courses within the past four years (2004-2007), and you are not submitting a Premedical Committee Recommendation, letters should be sent by two professors in whose classes you were enrolled for credit. One of the two should be from a science instructor. If you are not sending a letter from a science instructor from whom you received course credit, a letter may be sent from your supervisor at a volunteer scientific research position, or a paid scientific research position.
# If you have been out of school for more than four years (since 2003), you may substitute references from other professionals (i.e. employers).
Straight from the website, hope that helps.