When Apple Music launches, it will be backed by most of the current iTunes catalog, along with a digital first from Dr. Dre himself. The prolific musician and producer's debut studio album, The Chronic, is set to get its first-ever digital release on Apple's service. The album, originally released in 1992. That this is its first appearance on a digital service is thanks in part to Dre's legal battles with labels over re-releases of the album.
From Rolling Stone:
The album's absence could be explained, in part, over the rapper's legal battle over The Chronic with his former label Death Row Records. In 2011, Dre won back the digital rights of The Chronic after a judge ruled that the then-incarnation of Death Row could not sell the album digitally and granted Dre 100 percent proceeds of all Chronic online sales; Death Row retained the rights to The Chronic on all physical mediums.
Source: Rolling Stone