After the war ended, Tjio went to the Netherlands, whose government provided him with a fellowship for study in Europe. He worked in plant breeding in Denmark, Spain and Sweden. From 1948 to 1959 he did plant chromosome research in
Zaragoza in Spain and spent his summers in Sweden working with Professor
Albert Levan in Lund. In 1953, a lab mistake involving mixing
HeLa cells with the wrong liquid led Tjio and Levan to develop better techniques for staining and counting chromosomes. It allowed researchers for the first time to see and count each chromosome clearly in the HeLa cells with which they were working. They were the first to show that humans have 23 pairs of chromosomes rather than 24, as was previously believed. This was important for the study of developmental disorders, such as
Down syndrome, that involve the number of chromosomes. Tjio made his discovery of the correct human chromosome count (46 chromosomes, rather than 48 as counted in 1921 by
Theophilus Painter) in 1955 and the findings were published (with Levan as his co-author) in the Scandinavian journal
Hereditas on 26 January 1956. In 1958 Tjio went to the United States and in 1959 he joined the staff of the
National Institutes of Health in
Bethesda, Maryland. He received his Ph.D. in biophysics and cytogenetics from the
University of Colorado Denver. He spent the balance of his career at the NIH in human chromosome research. He was named scientist emeritus in 1992, but maintained a laboratory for the next five years. In 1997, he retired to
Gaithersburg, Maryland where he died in 2001 aged 82. ==Works==