increased isochromosome formation increased isochromosome formation with preferential breakage in the imr region A cell phenotype in which isochromosome formation occurs more frequently than in wild type cells, and in which the break point is located in the imr region in the majority of cases. In isochromosome formation, one chromosome arm is lost and the other arm is duplicated in its place. 2012-11-14T14:11:30Z FYPO:0001743 In S. pombe, isochromosome formation is most commonly observed with chromosome 3. When wild type cells form isochromosomes, the break-point is formed half the time in the imr region and half the time in the otr-irc region. midori increased occurrence of isochromosome formation with preferential breakage in the imr region