CD99 is a 32-kDa cell surface molecule presenton thymocytes, peripheral T cells, many other hematopoieticstem cells and somatic cells wereimplicated in cell-cell adhesion and cell-activationphenomena. Two major subtypes have been identifiedso far, designated CD99 type I and type II. We have investigated the correlation between thedegree of neural differentiation and the expressionof CD99 subtypes in three differentially differentiatedcell lines such as CADO-ES1, RD-ES, and SH-N-SY5Y, in order of differentiation. In addition,we induced differentiation of the RD-ES cellline by (6),2'-dibutyryl-cAMP (db-cAMP). Six daysafter treatment with db-cAMP, RD-ES cell line haschanged its morphology from uniform round cellsto cells with neurites, and initially CD99 typeII-overexpressed RD-ES cells showed significantdown-regulation of CD99 type II, whereas CD99type I expression remained constant. When RD-ES cells were transfected with the cDNA encodingfor CD99 type I-green fluorescence protein (GFP)and type II-GFP, CD99 type II transfected RD-EScell line remained unchanged with morphology ofundifferentiated form. Our data suggest that CD99type II acts as a negative regulator in the neuraldifferentiation of precursor cells that might occurduring nerve system development.