The Mark of Cain is a documentary about Russian prison tattoos. Meet the Slaves, the lowest caste of criminals who often have the word Slave tattooed on their face against their will, the Goats, prisoners who work with the guards and often end up spending long periods of time in solitary confinement to avoid physical contact with the other inmates, and the Thieves-in-Law, the highest ranking caste of criminals, whose tattoos tell the stories of their violent lives.
Here’s an interesting page about the symbolism of some of the tattoos.