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The polymorphic –219T/G variant in the APOE promoter has been associated with variations in basal transcriptional activity as well as with the risk of developing Alzheimer’s disease, myocardial infarction and early-onset coronary heart disease. The molecular mechanisms underlying these effects are presently unknown. In this report, we show that nuclear extracts from Jurkat cells form a T-specific complex with a motif including the –219 site within the APOE promoter. By DNA-affinity chromatography and mass spectrometry, the human heterogeneous nuclear ribonucleoprotein hnRNPA1(A1) was identified as one component of the complex. In vitro binding analysis indicated that a fragment of A1 had a marked binding specificity for the T form. Interaction of A1 with this region is driven by an adjacent telomeric-like sequence; however, the presence of G, but not T, at –219 position inhibited this interaction. The differences in transcriptional activity between the –219T and –219G promoter allelic forms correlated with the expression levels of A1 in several cell lines; also, over-expression of A1 increased the activity of the T form relative to that of the G form. These results indicate that A1 transactivates APOE promoter activity by direct and specific interaction with the –219T site., This work was supported by CICYT SAF 2000-0178 and 08.5/ 0065.1/2001 grants from Spanish Ministerio de Ciencia y TecnologõÂa and from Comunidad AutoÂnoma de Madrid, and by an institutional grant by FundacioÂn RamoÂn Areces to CBMSO.