A major goal in retroviral-based gene therapy is to establish methods that allow for the selection and tracking of transduced cell populations. Ex vivo gene marking of normal and malignant hemopoietic cells allows the cells to be followed subsequently in vivo. For in vivo applications, a neutral marker gene that is nonimmunogenic is desirable. To track two distinctively treated cell populations in a single individual, we designed and constructed two retroviral vectors; both of these vectors encode a truncated form of the human low-affinity nerve growth factor receptor, a neutral gene that does not transduce signals and is expected to be nonimmunogenic in humans. The two vectors, named Frape-1 and Frape-3, are identical at the protein level but differ at the DNA level, containing restriction sites that allow easy detection by polymerase chain reaction analysis. We show that cell lines and primary CD34+ cells can be readily transduced with these vectors and that transduced cells can be distinguished by polymerase chain reaction- and vector-specific restriction sites. These vectors will be useful for toxicity studies on in vivo gene therapy and for determining the source of relapse in hematological malignancies.