DNA Diagnostics Center (DDC):

• Life Sciences Building, Suite 1114, 700 South 10th Avenue, Minneapolis, MN 55415
• 1-800-613-5768 (this is the national line)
• There are a few different versions of the paternity test (which you can use to determine zygosity of twins) including Legal testing and Home testing.  They are the same test, but the Legal test needs to be administered in a lab or hospital. 

Twin Zygosity Testing