This vellum manuscript, dating to the fourth century C.E., originally contained the text of the whole Bible in Greek. It is the only manuscript written in uncial script that preserves the entire Christian Greek Scriptures intact. (See MANUSCRIPTS; UNCIAL.) However, almost all of the Hebrew Scriptures prior to Ezra 9:9, as well as some later portions, are missing. Thus, just over 400 leaves of an estimated total of at least 730 are known to exist.

Little is known about the origin of this codex. Some suggest that it was produced in Rome, Caesarea, or Alexandria. It was eventually stored in the library at the Monastery of St. Catherine, located at the foot of Mt. Sinai. It came to the attention of European scholars in the 19th century, and much of the codex was taken to other libraries. Parts of the manuscript are now found in four different institutions, including the monastery at Mt. Sinai, but most of it is located in the British Library in London.

Scholars have used this authoritative manuscript, along with Codex Vaticanus and Codex Alexandrinus, to identify and correct additions and scribal errors that were introduced into later Bible manuscripts.​—See Media Gallery, “Codex Sinaiticus​—End of Mark’s Gospel.”