Fanconi anemia (FA) is an autosomal reces- sive cancer susceptibility syndrome with at least eight comple- mentation groups (A-H). Two FA genes, corresponding to complementation groups A and C, have been cloned, but the function of the FAA and FAC proteins remains unknown. We have recently shown that the FAA and FAC proteins bind and form a nuclear complex. In the current study, we analyzed the FAA and FAC proteins in normal lymphoblasts and lympho- blasts from multiple FA complementation groups. In contrast to normal controls, FA cells derived from groups A, B, C, E, F, G, and H were defective in the formation of the FAAyFAC protein complex, the phosphorylation of the FAA protein, and the accumulation of the FAAyFAC protein complex in the nucleus. These biochemical events seem to define a signaling pathway required for the maintenance of genomic stability and normal hematopoiesis. Our results support the idea that multiple gene products cooperate in the FA Pathway. interaction, mediated by other adaptor proteins. Regulated posttranslational modifications of the FAA or FAC protein, such as phosphorylation, may also be required for interaction of the FA proteins. It is not yet known whether the FAAyFAC binding interaction is constitutive or inducible under various cellular conditions or stresses. Interaction between the FAA and FAC proteins has func- tional importance. A patient-derived missense mutation in the FAC protein, FACL554P, prevents the formation of the FAAyFAC complex (11), suggesting that protein binding may be required for the normal (non-FA) cellular phenotype. Whether the interaction of FAA and FAC is required for nuclear translocation of the protein complex also is currently unclear. The expression and binding of FAA and FAC in the various FA complementation groups has not yet been exam- ined. To explore the functional importance of the FAAyFAC protein interaction further, we analyzed FAAyFAC binding, FAA phosphorylation, and FAAyFAC nuclear accumulation in lymphoblast lines derived from normal adult controls or patients with FA. Our results show that the FAA protein is phosphorylated in normal cells and FA-D cells but not in FA cells derived from groups A, B, C, E, F, G, and H. Moreover, the phosphorylation of FAA correlated with both the binding of FAA to FAC and the nuclear accumulation of FAA and FAC. Our results suggest that FAA phosphorylation is an important regulatory event controlling the FA signaling path- way.