Smaller hair bulbs and less Mx proliferation of Notch1‐deficient HFs have been attributed to paracrine Igfbp3 induced in the DP.11 However, we found no discernible difference in levels of Igfbp3 protein between control and Hes1eKO DPs in both anagen and telogen phases (Figure S2B). Here, NOTCH1 is linked to Hand-foot syndrome.