SIX2 and Hypercalcemia: Similar to Six2-Cre;Sik1fl/+;Sik2fl/fl;Sik3fl/fl mice, only Six2-Cre;Sik2fl/fl;Sik3fl/fl mice showed increased Cyp27b1 expression and serum 1,25-vitamin D levels, mild (but statistically significant) hypercalcemia, and PTH suppression (Figure 6 and Supplemental Figure 10, D–H).