Harry Potter and the Half Blood Prince: Horcrux?

Regulus A. Black, Sirius's Death Eater brother. In the novel, when they are cleaning, they come across a locket that they are unable to open. That could be the locket.

I think everyone thinks that R.A.B is Regulas. The locket they found while they were cleaning in OOTP is the locket. In Half Blood Prince Mundungus Fletcher was showing the barman some stuff he got from Sirius's house. My guess is the locket is now with the Bar Man from Hog's Head. That vey barman is also the brother of Albus Dumbledore, Aberfort Dumbledore. So the locket is with DD's Brother.

I think it may be Regulus Black but that seems to easy. J K Rowling has never spoon fed plot and Regulus was handed to us on a silver platter. I think it may be a character who until now has only been mentioned once or twice who will end up gaining notoriety in the seventh book.

I agree it will probably be Sirius' brother. Sirius explained that while he was in Azkaban, that his brother was killed by Death Eaters. And yes, the locket seems to be originally found when they were cleaning.

However, nothing has been confirmed and only JK and a few select people know. The anticipation is killing me.