The punishment of Cain is the first "racial" verse in the Bible.  Not that there is anything explicit in the text that has to do with race:  rather, later interpreters inserted racial speculations into the story.
    
God punished Cain by putting a curse on him, making him a wanderer:

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        "When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth" (Gen 4:12).

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Cain's talent had been in farming, and God took this ability away from him.  Cain showed no repentance for his action, but only protested that his punishment was too severe.  Not only was he to be driven away from his home to become a wanderer, but he was liable to be killed by anyone he met.  And, most important, he said that he was to be hidden from God's presence (Gen 4:14).

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This is the most severe consequence of sin, Cain's or our own:  it breaks relationship with God. Adam and Eve had already been cast out of the Garden, but now Cain is cast out even further!  He has compounded the sin of his parents (which was against God), by adding the sin against his own brother.  So, for Cain, there is a double alienation, a double disowning.  Not only is he cast out of God's presence, but also out of his family.  And the saddest part of it is that this doesn't bother him so much as the possibility that strangers might kill him.

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This is where the "mark of Cain" comes in.

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         "Then the Lord put a mark on Cain so that no one who found him would kill him" (Gen 4:15).

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This mark has been a source of endless and fantastic speculation:  what kind of mark and where was it put?  A tattoo?A brand? A style of hair? A skin disease that caused people to shun him?  This is also where racism intrudes.  Certain late traditions, not those of the rabbis or Church Fathers, suggested that the mark was dark skin.  This led to the convenient conclusion that Africans were descendants of Cain and inherited his curse.  Thus they were an inferior race and fit only for subjugation and servitude.  The mark of Cain became one of the two Biblical props for the justification of slavery in America.  The other one was, of course, Noah's curse on Canaan (see below).

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This is nonsense.  First of all, there are problems with the translation of the text.  The literal translation of this verse reads,

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        "And the Lord appointed a sign for Cain , so that no one finding him would slay him" (NAS).

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The word "mark" is elsewhere translated "sign," and is used many times in the Old Testament.  The preposition "on" used in Hebrew is usually translated "for" or "to."  So the verse could be interpreted as God gave Cain a sign of divine protection, without actually doing anything to his body.

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         The assertion that the mark (sign) of Cain was dark skin is false for six reasons:

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        1.  This teaching reads into Scripture what isn't there:  there is no mention of skin color.
        2.  It makes the mark genetic, but Scripture doesn't say that the mark was passed to Cain's descendants
        3.  Cain's descendants all died in the Flood, so the mark could not be passed on to later generations.
        4.  The mark was not the curse.  The curse was that the ground would be unfruitful and that Cain would be a  restless wanderer, apart from the presence of the Lord (Gen 4:11-12, Gen 4:16).   The mark was a seal of divine protection.
        5.  Unlike Noah's curse on Canaan, there is no mention that Cain would be a servant or slave to other men.
        6.  Even if the mark was dark skin, and was genetic, and his descendants managed to survive the Flood, the  racist interpretation of this passage ignored the significance of the mark:

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        "If anyone kills Cain, he will suffer vengeance seven times over" (Gen 4:15).

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In the context of the 18th and 19th Centuries, this would have meant that any slave-trader or slave-owner who killed a slave was liable to a 7-fold judgment of God.  The expositors of the time chose to ignore this provision, because instead of upholding the system of slavery, it would have overthrown it.

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19th Century exposition of the mark of Cain is a prime example of culture corrupting Scripture.  When the religious leadership of a society distorts Scripture to legitimize an evil practice, it is not simply a mistake or a failure of scholarship.  By reading into the Bible the sins of their time, they attribute their evil practices to God's will, which is nothing short of demonic.