Presented here are hypothetical findings from studies of heterokaryons formed from seven human xeroderma pigmentosum cell strains: XP1 XP2 XP3 XP4 XP5 XP6 XP7 XP1 - XP2 - - XP3 - - - XP4 + + + - XP5 + + + + - XP6 + + + + - - XP7 + + + + - - - Note: + = complementation; - = no complementation These data are measurements of the occurrence or nonoccurrence of unscheduled DNA synthesis in the fused heterokaryon. None of the strains alone shows any unscheduled DNA synthesis. Which strains fall into the same complementation groups? How many different groups are revealed based on these data? What can we conclude about the genetic basis of XP from these data?