This is straight from their secondary application:
"6) LETTERS OF RECOMMENDATION: Submit a Premedical Committee recommendation or letters coordinated by the Premedical Advisor at your undergraduate school. If you do not have a Premedical Advisor or Committee, a minimum of three letters are required from faculty (senior professors) who can provide a well considered evaluation of your qualifications for the medical profession. (Two of the three must be from faculty at a four-year undergraduate institution in either the Departments of Biology or Chemistry. The third recommendation must be in humanities or social sciences. If you are currently in a graduate program, the third letter may come from your graduate professor.) A MAXIMUM OF FIVE LETTERS WILL BE ACCEPTED. Also, if you are applying for the MD/PH.D program you will need to provide us with a separate packet of recommendation letters. Recommendations mailed prior to our request will need to be resubmitted."