(ESV)"The beginning of the gospel of Jesus Christ, the Son of God."
(NIV)"The beginning of the good news about Jesus the Messiah, the Son of God,"
(NRSV)"The beginning of the good news of Jesus Christ."
We notice a few differences with these translations at Mark 1:1. The ESV has gospel while the NIV and NRSV have good news. The NIV has Messiah while the ESV and NRSV have Christ. Messiah and Christ both mean Anointed One.
But the biggest difference is that the ESV and NIV include the phrase, "Son of God" while the NRSV omits it. There is actually a textual variant in the manuscripts. A few of the manuscripts do not have "Son of God" but most of the manuscripts do include the phrase.
The omission of the phrase "Son of God" in a few ancient manuscripts is probably explained by a copyist error. The words "Son of God" also appear toward the end of the Gospel at Mark 15:39. So "Son of God" at 1:1 and 15:39 sort of serve as bookends. It is likely that the words "Son of God" are part of the original text at Mark 1:1.
(ESV) English Standard Version
(NIV) New International Version
(NRSV) New Revised Standard Version