Three of the four groups of letters below are built on one and the same rule. Which group is built differently? BDF, JLN, PRT, SUX

In BDF the letters stand at places 2, 4, 6, in JLN at 10, 12, 14, and in PRT at 16, 18, 20 — each group moving on by two places and then two again. SUX stands at 19, 21, 24: two places and then three, which breaks the rule. BDF looks singular only because it opens the alphabet, JLN only because it lies in the middle, and PRT only because its letters are the ones nearest to those of SUX — none of these has any bearing on the rule the groups are built on.