Complex sentence: He was working in his office at the AMC Cancer Research Center until days before he died suddenly in San Francisco on November 24,1990 of a malignant brain tumor diagnosed only days before while visiting his daughter in Berkeley, California.
Split into simpler sentences: He was working in his office at the AMC Cancer Research Center until days before he died suddenly in San Francisco on November 24,1990 of a malignant brain tumor. The tumor was diagnosed only days before while Cullen was visiting his daughter in Berkeley, California.