The study describes the spontaneous transformation of human keratinocytes from adult skin into a cell line named HaCaT, which exhibits immortalization and a transformed phenotype in vitro but remains nontumorigenic. HaCaT cells retain their differentiation capacity, forming an orderly structured and differentiated epidermal tissue when transplanted onto nude mice. The cells express differentiation-specific keratins (keratins 1 and 10) and other markers ( involucrin and filaggrin). Despite altered growth potential, HaCaT cells are the first permanent epithelial cell line from adult human skin that maintains normal differentiation. The line is aneuploid with stable marker chromosomes, and DNA fingerprinting confirms its identity with the tissue of origin. This cell line provides a valuable tool for studying the regulation of keratinization in human cells and the role of oncogenes in malignant transformation.The study describes the spontaneous transformation of human keratinocytes from adult skin into a cell line named HaCaT, which exhibits immortalization and a transformed phenotype in vitro but remains nontumorigenic. HaCaT cells retain their differentiation capacity, forming an orderly structured and differentiated epidermal tissue when transplanted onto nude mice. The cells express differentiation-specific keratins (keratins 1 and 10) and other markers ( involucrin and filaggrin). Despite altered growth potential, HaCaT cells are the first permanent epithelial cell line from adult human skin that maintains normal differentiation. The line is aneuploid with stable marker chromosomes, and DNA fingerprinting confirms its identity with the tissue of origin. This cell line provides a valuable tool for studying the regulation of keratinization in human cells and the role of oncogenes in malignant transformation.