HOX A5 overexpression inhibits the butyrate-induced differentiation of K562 cells. K562 cells were transfected with FLAG epitope-tagged HOX A5 (pFAT) or the empty vector (pCMV-thy-1). Erythroid differentiation was measured by heme content and cell surface expression of glycophorin A 72 hours posttransfection. Heme content was determined by direct benzidine staining of cells grown with or without butyrate; the data represent a mean of seven experiments. Glycophorin A was measured by FACS analysis and scored as the percentage of transfected cells displaying specific fluorescence intensity above background fluorescence levels; the data represent the mean of three experiments. Monocytic differentiation was measured by nonspecific esterase positivity of cells treated for 72 hours with or without 8 nmol/L TPA, and stained cytospun cells were scored by light microscopy; the data represent the mean of two experiments. *P < .001; **P < .05.