Contribution No. 24–64, Department of Biology, City of Hope Medical Center. This work was supported in part by grant CA-05138-04, National Cancer Institute, U.S. Public Health Service. This project was begun during a visit to this laboratory of W. Beçak and M. L. Beçak of the Seccão de Genética, Instituto Butantan, Sao Paulo, Brasil.
The editorial assistance of Patricia A. Ray is gratefully acknowledged.