{The sign of the Son of Man in heaven} (	o s(886d)eion tou
huiou tou anthr(9370)ou en ouran(9369)). Many theories have been
suggested like the cross in the sky, etc. Bruce sees a reference
to  Da 7:13  "one like the Son of man" and holds that Christ
himself is the sign in question (the genitive of apposition).
This is certainly possible. It is confirmed by the rest of the
verse: "They shall see the Son of man coming." See  Mt 16:27  26:64 . The Jews had repeatedly asked for such a sign (Broadus)
as in  Mt 12:38  16:1  Joh 2:18 .