We examined the expression levels of the four HCN isoforms (HCN1‐4) using cell lines from publicly available TCGA database, and on breast cancer patient biopsies by IHC and found that only HCN2 and HCN3 are overexpressed in breast cancer cells (Figure 1A‐D and Supporting information Figure S1 and S2). Here, HCN3 is linked to breast cancer.