1. Letter from Leon A. Heppel to Maxine Singer Creator: Heppel, Leon A.National Institutes of Health (U.S.) Date: 13 December 1955 Genre: Letters (correspondence)
2. Letter from Leon A. Heppel to Maxine Singer Creator: Heppel, Leon A.National Institutes of Health (U.S.) Date: [28 December 1957?] Genre: Letters (correspondence)
3. Letter from Leon A. Heppel to Maxine Singer Creator: Heppel, Leon A.National Institutes of Health (U.S.) Date: [27 July 1958?] Genre: Letters (correspondence)
4. Letter from Leon A. Heppel to Maxine Singer Creator: Heppel, Leon A.National Institutes of Health (U.S.) Date: 27 October 1955 Genre: Letters (correspondence)