The fact that the Antiochene and Roman sources were reproduced by both Evangelists Matthew and Luke was due to the importance of those Churches.

Streeter hypothesized a proto-Luke document, an early version of Luke that did not incorporate material from Mark or the birth narrative.

According to this hypothesis, the evangelist added material from Mark and the birth narratives later.

[3] A contemporary version of the four-source theory omits proto-Luke, with the evangelist combining Mark, Q, and L directly.

[4] Still, the gospel might have circulated originally without the birth narrative in the first two chapters.