EFNB1 and hydrops fetalis: Corroborating the HF progression in CM-specific Efnb1 KO mice, while T-tubule misalignment from Z-lines (HFrEF marker) is already detectable but limited to discrete local regions in the cardiac tissue of some KO CMs at 2 months of age, this disorganization progressively spreads over time to all CMs after 12 months compared with WT mice (Figure 8F).