To map the cellular site where Loxl2 functioned to trigger HF, we performed tissue-specific knockout of Loxl2 in cardiac fibroblasts, using the promoter of Tcf21—a transcription factor specifically expressed in fibroblasts18—to drive the expression of Cre recombinase (CreERT) whose enzymatic activity is inducible by tamoxifen (iTcf21CreERT; Supplementary Fig.5a,b). Here, TCF21 is linked to hydrops fetalis.