To exclude the effects of anaemia-related hypoxia on chondrocyte survival, the Col2a1-CreERT2 mice were crossed with HbbF/F mice to produce chondrocyte-specific tamoxifen-induced Hbb knockout (Hbb-cKO) mice that exhibited no anaemia or hypoxia in other tissues, such as the liver and muscle (Extended Data Fig. 9a–c and Supplementary Fig. 10), and died within 7 days after birth (Extended Data Fig. 9d). Here, COL2A1 is linked to anemia (phenotype).