Micro-hardness test of incisors of 4-week-old mice. (A) Light micrographs taken from the indentation sites in the incisal enamel of WT, S55A Het, and S55D Het mice. De, dentin. En, enamel. Scale bar, 100um. (B) Statistic comparison of micro-hardness of incisal enamel at inner, middle and outer locations between WT, S55A Hets and S55D Hets. S55A Hets showed significantly lower enamel hardness than S55D Hets and WT. S55D Hets also showed decreased enamel hardness compared to WT. Both S55A and S55D Hets showed lower dentin hardness than WT. * P<0.05. ** P<0.01. *** P<0.005. **** P<0.001.