Abraham 1

 In the Books of Moses and Abraham, we learn that the Cain and his posterity was cursed and forbidden to hold the priesthood because of their wickedness, as we read - 

"Now the first government of Egypt was established by Pharaoh, the eldest son of Egyptus, the daughter of Ham, and it was after the manner of the government of Ham, which was patriarchal.

26 Pharaoh, being a righteous man, established his kingdom and judged his people wisely and justly all his days, seeking earnestly to imitate that order established by the fathers in the first generations, in the days of the first patriarchal reign, even in the reign of Adam, and also of Noah, his father, who blessed him with the blessings of the earth, and with the blessings of wisdom, but cursed him as pertaining to the Priesthood."  (Abraham 1:25–26)

I can't help but ask, does that mean all the posterity of Cain were wicked.  Absolutely not!  Obviously, Egyptus, wife of Ham, must have been righteous, and I assume her daughter Egyptus was righteous because the first Pharoah was righteous.

I assume that because of Cain and much of his wicked posterity, that it must have been understood from the great patriarchs, that the priesthood was at that time to be withheld from Cain's posterity.

How long was this withholding to take place?   I don't know.  But I do know that there were many of Cain's posterity that repented and were faithful.