CPA3 and osteoarthritis: Given that several of the Cpa3-Cre;Mcl-1fl/fl mice developed osteoarthritis (Figure 2—figure supplement 1b) and the Cpa3-Cre;Mcl-1fl/fl mast cell deficiency is known to be incompletely penetrant with the presence of residual mast cells observed in certain organs and mice (Lilla et al., 2011; Reber et al., 2012; Tsai et al., 2005), we performed additional anti-tryptase immunostaining of the joint tissues from the Cpa3-Cre;Mcl-1fl/fl mice that developed osteoarthritis following DMM to more comprehensively characterize these mice and their stifle joints for mast cell deficiency.