I had read this in a book from Zecharia Sitchin and this book was "The lost book of Enki". This book is the story about Enki a Sumerian God which could be one of the Gods of the Genesis, it is a copy, translation from some writings of the Sumerian and a part of fiction because in Sumerian books are books mentioned which are lost until now.There he wrote "Ningishzidda ( the Egyptian god Thoth) Mardoks younger brother, then genetically alters Ka-In so that he not grow facial hair - - this is "the mark of Cain". It is a reference to the bible about the mark of Cain. That is all.I did some research for myself and came to the conclusion, nobody knows exactly which mark was put on Cain is it blackened skin is it a sign on the forehead, nobody knows it.Perhaps Sitchin has right, but if not... ?! It is a very interesting and gripping book, even if there is not all is a fact.