BBC World Service's The World Today programme is asking migrants who have been successful in their adopted countries how they got to the top of their field.
Susumu Tonegawa is a professor at the Massachusetts Institute of Technology. He won the 1987 Nobel Prize for Medicine. He was born and grew up in Japan.
At the age of 22 I left Japan to do graduate study at the University of California, San Diego.
I wanted to become a so-called molecular biologist - this is many years ago, 1963 - and at that time there were no real molecular biology laboratories in Japan, and my professor told me I should go abroad to get the proper training.
My philosophy has always been that I choose the best environment in order to get my work done
1987 - I was awarded the Nobel Prize for Physiology and Medicine for my research in immunology.
If I had stayed in Japan, probably I could not have done the work that I did.
The main reason is because the world for which the Nobel Prize was given to me was for studies I did in my 30s - between 31 and 37 - and in the Japanese traditional system, it is very rare that any young scientist of that age group would be given the total independence and resources to carry out the research based on his or her own ideas and abilities.
I value the Japanese culture, customs and traditions in many respects, but at the same time I have already taken the view, since I was young, that science is totally international.
Wherever you are, we are basically working under the same criteria - in other words, we all want to understand how nature is.
It's a totally international thing to do. It cannot be that what is true inside of Japan in science will not apply outside of Japan.
For that matter, I have always taken the view that wherever you are in the world, you should always be able to do science.
My philosophy has always been that I choose the best environment in order to get my work done.
As to the family, raising children and so on, there are different factors involved.
I have three relatively small children - aged 10 to 15. They were born in the United States.
It's very interesting the way they grow - they are totally American in one respect, but also they have maintained a Japanese tradition, primarily due to the influence of their parents.
My position is that as time goes by, more and more people who have a cosmopolitan sense of barriers are the ones who will have a more interesting life.
I don't really feel that I have sacrificed anything specific with respect to my past career.
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