IL-17, primarily produced by CD4+ Th17 cells and certain innate lymphoid cells, is a key proinflammatory cytokine implicated in various inflammatory diseases, including psoriasis, atopic dermatitis, hidradenitis suppurativa, alopecia areata, pityriasis rubra pilaris, pemphigus, and systemic sclerosis. The gene discussed is CD4; the disease is systemic sclerosis.