FAC protein is located in the cytoplasm and nucleus of normal human lymphoblasts. JY cells were lysed by dounce homogenization and separated into subcellular fractions according to the method of Lewis et al.26 Individual fractions were separated by SDS-PAGE and immunoblotted with FAC1 (A) or β tubulin (B) antiserum. Lane 1, plasma membrane fraction; lane 2, mitochondrial fraction; lane 3, nuclear fraction; lane 4, cytoplasmic fraction. The cytoplasmic fraction contains one fourth the cell number contained in lanes 1 to 3. Similar results were obtained with FAN2 antiserum (data not shown). Values for quality of fractionation were calculated by desitometric analysis. The total amount of signal observed for β tubulin (a cytoplasmic marker) in all fractions was measured and then used to calculate the amount occurring in the nuclear fraction as an artifact.