She who is in Babylon, chosen together with you, greets you; and so does Mark, my son.
And so doth Mark my son ... Peter was Mark's mentor, not his actual father; and he is called "my son" in the sense that Paul thus referred to Timothy.  It is almost universally agreed that this is the John Mark of Acts, who is the author of the second Gospel.  See the introduction to Mark in my Commentary on Mark for a full discussion.