The 12-marker test is $99 + about $2 for mailing the kit to you. The 25-marker test is $169 + about the same for mailing. If you choose the 12-marker test and then decide to upgrade it to 25-marker (same swab kit), the cost is $90 additional (total of $189). If we choose the 25-marker test, we would save $20 over doing the upgrade later. What is the difference: 12 markers are sufficient to determine whether or not two people are related. The value of the 25 marker test occurs when two participants are related based on the 12 marker test. The objective of the upgrade to the 25 marker test is to reduce the time frame of the common ancestor between the matching participants. Two individuals are considered related for the 12 marker test if they match 12/12, 11/12, and sometimes with 10/12. In the first two situations, an upgrade to the 25 marker test is recommended. I have no preference as to which marker set we do in our initial testing. I would guess that individually we can make different choices. Some have suggested that because THOMAS is such a large population that we might end up using the 25 marker more often. I'm with the group on this decision. Bonnie