I just listened to the Moody Blues' In Search of the Lost Chord for the first time. I'm not sure, but I think they quote part of Gershwin's "Rhapsody in Blue" in "House of Four Doors (Part 1)." At around 3:15, a melody emerges from the rumbling. As far as I can figure out, the notes are C# D E A. What I think is a corresponding melody occurs in "Rhapsody in Blue" at around 11:30 in the version I have by the Royal Philharmonic Orchestra. I'm pretty sure that it's G# A B and then E and B (an octave lower) simultaneously. Simplified to representative notes of the scale, it's the third, the fourth, the fifth, and the tonic (plus the fifth in the case of "Rhapsody in Blue," if I have it correct). These two sections are either in the same rhythm or a very similar rhythm, and after each, the melody continues to rise.
According to the Wikipedia page, each section in the song is supposed to represent an era of music, so perhaps it's a conscious nod to Gershwin.
