A recent tweet by writer dream hampton states that Jay Electronica and stic.man of dead prez were both ghostwriters on Nas' Untitled album. The statement was supported by Rappers I Know, who shared a story about a phone call received from Jay Electronica in 2007 saying that he was ghostwriting for Nas. See dream's tweet below, and read the Rappers I Know story here.
[via Rappers I Know]
Update. In this interview below from last week, Nas denies using ghostwriters.
