As Tony notes in his answer there is one article in Mark 1:1:
ἀρχὴ τοῦ εὐαγγελίου Ἰησοῦ Χριστοῦ υἱοῦ θεοῦ
The Greek article does much more than to make definite as the English article. However, if one wanted to approach the phrase from this view, then the use of a single article before Gospel means what follows is the specific Gospel, Jesus-Christ-Son-God. The one article eliminates the need for any other because it is not the Gospel about Jesus Christ the Son of God; It is the Gospel... When one reads the Gospel, one learns Jesus is the Christ who is the Son of God.
In addition, there is no article before ἀρχὴ "...showing that the expression is a kind of title. It is 'the beginning,' not of his book, but of the facts of the Gospel."
1Using the article with Gospel functions to reinforce the titular nature of ἀρχὴ.
A similar use of a single article occurs in 15:39:
And when the centurion, who stood facing him, saw that in this way he breathed his last, he said, “Truly this man was the Son[i] of God!” [ESV]
i. Mark 15:39 Or a son
ἰδὼν δὲ ὁ κεντυρίων ὁ παρεστηκὼς ἐξ ἐναντίας αὐτοῦ ὅτι οὕτως ἐξέπνευσεν εἶπεν ἀληθῶς οὗτος ὁ ἄνθρωπος υἱὸς θεοῦ ἦν
The phrase is literally, the man Son God was. As with the Gospel, the construction gives emphasis on the man leaving open the possibility the meaning is not the exclusive Son of God. [The ESV translator notes the phrase could mean a Son of God.] However, any ambiguity in meaning is of little significance because the source of the statement is the Roman centurion, not the Gospel writer. The lack of the article could be a result of "poor" Greek on the part of a Roman, or it may in fact reflect the Centurion's belief Jesus was "a" son of god based on his beliefs in his gods.
The Centurion's witness is not to prove Jesus' identity. It is to contrast his reaction with that of the Jewish people and particularly of the chief priests and scribes:
29 And those who passed by derided him, wagging their heads and saying, “Aha! You who would destroy the temple and rebuild it in three days, 30 save yourself, and come down from the cross!” 31 So also the chief priests with the scribes mocked him to one another, saying, “He saved others; he cannot save himself. 32 Let the Christ, the King of Israel, come down now from the cross that we may see and believe.” Those who were crucified with him also reviled him. (Mark 15)
Where the people with the knowledge of the true God mocked and scorned Jesus, the pagan(?) Centurion believed He was deity.
1. Vincent's Word Studies