How old is a polymorphism?

This publication has 6 references indexed in Scilit:

• The origin of MHC class II gene polymorphism within the genus Mus
  Nature, 1988
• Structure of the human class I histocompatibility antigen, HLA-A2
  Nature, 1987
• Origin of major histocompatibility complex polymorphism: The trans-species hypothesis
  Human Immunology, 1987
• Immunology: The ins and outs of antigen processing and presentations
  Nature, 1986
• Murine Major Histocompatibility Complex Class-I Mutants: Molecular Analysis and Structure-Function Implications
  Annual Review of Immunology, 1986
• DNA sequence analysis of a rat RT1 class 11 A ? gene
  Immunogenetics, 1985