Compared to primary keratinocytes, HaCaT cells are easier to transfect and yet still maintain at least some features of normal epidermal proliferation and differentiation. This chapter describes methods used in our laboratory to maintain HaCaT cells in an undifferentiated state and to use the siRNA technology to efficiently deplete a gene product of interest from these cells. We also provide protocols on how to couple siRNA knockdown with a clonal assay to examine keratinocyte proliferation potential and a luciferase reporter assay to examine promoter regulation.