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Catholic Encyclopedia
http://www.newadvent.org/cathen/09682a.htm
SS. Mark and Marcellian, blood brothers martyred at Rome in the Diocletian persecution, probably in 286.
For All the Saints
http://www.saintpatrickdc.org/ss/0618.htm#mark
Essay on SS. Mark and Marcellian, twins, both of whom were married deacons.