ὁ μονογενὴς θεός is the reading of the Sinaitic and Vatican mss. in John i. 18.  The insertion of the words οὐδὲ ὁ υιἰος, adopted by R.V. in Matt. xxiv. 36, but of which St. Basil knows nothing, as appears from his argument on the difference between the statements of St. Matthew and St. Mark on this subject in Letter ccxxxvi., is supported by these same two mss.