Alma 3:7-8
Laman and Lemuel were cursed for trying to kill the prophets among them. Therefore a mark was set upon them and their posterity and also upon the sons of Ishmael and all their wives so that the people of the Lord would be set apart and so that they and their seed would not mix traditions with their unbelieving brethren. The mark was not the curse, it was a preservation for the people of the Lord by making the two people visibly distinguishable. It was designed so that those without the curse would not desire to mix with those who had been cursed for their wickedness or raised with false traditions.