Doctrin and Covenants 29:36-41.

     Shall I tell you the law of God in regard to the African
     race? If the white man who belongs to the chosen seed mixes
     his blood with the seed of Cain, the penalty, under the law
     of God, is death on the spot.

     This will always be so.  Cain slew his brother... and the
     Lord put a mark upon him, which is the flat nose and black
     skin.

     That curse will remain upon them, and they can never hold
     the Preisthood or share in it untill all other descendants
     of Adam have received the promises and enjoyed the blessings
     of the Priesthood and keys therepf.

     When all other children of Adam have had the privelege of
     receiving the Priesthood, and of comming into the kingdom of
     God, and of being redeemed from the four quarters of the earth,
     and have received their resurrection from the dead, then will
     be time enough to remove the curse from Cain and his posterity."

All of this however is the least of the problems with the Mormon
religion.  If you knew what happened once you got in so far that you
could not turn back (This is after taking the oath of death...) you
would laugh, then you would probably say, "That's insane... How could
people believe this."