A variety of molecular techniques can be used to investigate diseases by analyzing protein, DNA, and RNA. Flow cytometry, PCR, histology and immunohistochemistry are assays which can identify specific cell populations and provide valuable information regarding the pathologic characteristics of those populations. Flow cytometry analyzes the surface markers of cells. Histology and immunohistochemistry further characterize surface and cellular molecules and aid in the diagnosis of certain skin diseases. Gene expression profiling allows investigators to examine the genes detectable to determine the function of the cells involved, and PCR techniques are useful for the diagnosis of certain conditions and for DNA analysis.