DNA
From Tardis Wiki, the free Doctor Who reference
DNA (Deoxyribonucleic acid) is the nucleic acid that serves as the basis for carbon-based life. The blueprint for a living organism, it codes for proteins. (DW: School Reunion)
While Humans have two strands, Time Lords have three strands, and can have one set of strands for each regeneration. (DW: The End of the World). According to the Doctor in Frontier Worlds he has four strands, but only three as visible to non-temporally aligned scanners (therefore part of his DNA is linked to the Time Vortex).
