Cofitness data is data representing the similarity of growth fitness under various conditions between any two different deletion strains. Under the assumption that strains lacking the diphthamide synthetase gene should have high cofitness with strain lacking other diphthamide biosynthesis genes,  they identified ylr143w as the strain with the highest cofitness to the all other strains lacking known diphthamide biosynthesis genes.
Cofitness data reflects how well different strains grow under various conditions. Assuming that strains without the diphthamide synthetase gene should have high cofitness with other strains lacking diphthamide biosynthesis genes, they identified ylr143w as the strain with the highest cofitness to all other strains missing known diphthamide biosynthesis genes.