In mice, there are two alleles: Thy1.1 (Thy 1a, CD90.1) and Thy1.2 (Thy 1b, CD90.2). They differ by only one amino acid at position 108; an arginine in Thy-1.1 and a glutamine in Thy-1.2. Thy 1.2 is expressed by most strains of mice, whereas Thy1.1 is expressed by some like AKR/J and PL mouse strains.