CLDN1 and Hand-foot syndrome: We were especially interested in Cldn-1 because (1) it is the most widespread TJ protein in HFs, (2) it shows a dynamic change during hair cycle, (3) it is known to be involved in proliferation19–21 and apoptosis22, which are important for hair growth and regression, (4) Cldn-1 knock-out in mouse and human results in an aberrant hair phenotype9,23, and (5) Cldn-1 is altered in the epidermis of skin diseases, such as atopic dermatitis (AD)20,24 and psoriasis25,26.