[5] In addition to having over 1,385 papers published on cancer and experimental cell research, Klein authored over 13 books in Swedish on a wide range of topics, including essays on the Holocaust in Hungary.

[8] Klein wrote in Pietà and elsewhere about his experiences during the Holocaust as a teenager in Budapest, after the German invasion of Hungary in March 1944.

Between May and July 1944, 437,000 Hungarian Jews were deported by cattle train to the Auschwitz concentration camp, to be "resettled", according to the Germans.

In May or June 1944, Klein was working as a junior secretary for the Jewish Council in Sip Street, Budapest, when he was shown a copy of the Vrba-Wetzler report by his boss, Dr. Zoltán Kohn.

[9] Decades later, he looked for Vrba, then a professor of pharmacology in Canada, to thank him, and subsequently wrote about him and his report in two essays: "The Ultimate Fear of the Traveler Returning from Hell" in Pietà (first published in Sweden in 1989), and "Confronting the Holocaust: An Eyewitness Account" (2011) in The Auschwitz Reports and the Holocaust in Hungary, edited by Randolph L. Braham and William vanden Heuvel.

When the war ended, Klein and a friend traveled to Szeged, a town 300 km from Budapest, to find out whether its university was still functioning.

[12][13] He returned to Budapest in September 1947 and married Eva, who joined him in Stockholm in March 1948,[14] shortly before the Hungarian People's Republic came into existence.

[2] In 1960 the Kleins published an important paper in Cancer Research, "Demonstration of Resistance against Methylcholanthrene-induced Sarcomas in the Primary Autochthonous Host".

[17] According to Klein's obituary in Nature, researchers at the time believed that cancers carried "a common antigen that the immune system could recognize.

[18] He was responsible, with Henry Harris, for establishing the "phenomenon of tumour suppression ... using the technique of somatic cell hydridization".