The defensins are an interesting protein family that is important in mammalian immune systems. It now seems that most mammals have some versions of the alpha- and beta-defensins, but only some primates have theta-defensins. In the human, there is a pseudo-gene for a theta-defensin that is post-translationally processed into a cyclic peptide called retrocyclin. It is possible that our loss of a functional retrocyclin contributes to our susceptibility to HIV and AIDS; its an interesting avenue for future gene therapy.
I can't find out too much about retrocyclin; since you pretty much have to use rhesus monkeys to study it, there isn't a lot out there yet. A good place to start is the OMIM entry for the alpha-defensins, and the paper by Cole et al.