"She who is at Babylon, who is likewise chosen, sends you greetings; and so does my son Mark" (1 Peter. 5.13)."She who is at Babylon" refers to the saints in Rome.
My study notes read as follows:
Mark, also called John Mark, was known to many of this letter's readers because he had traveled widely (Acts 12.25-13.13; 15.36-41) and was recognized as a leader in the church (Colossians 4.10; Philemon 1.24). Mark was probably with the disciples at the time of Jesus' arrest (Mark 14.51,52). Tradition holds that Peter was Mark's main source of information when Mark wrote his Gospel. Barnabas was Mark's cousin.