Human and mouse chromosomes are indicated in red and blue, respectively. Yellow arrows show pieces of apparatus, the so-call “centromere,” used to pull the chromosomes into both daughter cells during cell division. Normal human (a) and mouse (b) chromosomes are shown in the upper panels. The abnormal chromosomes comprise a mouse chromosome and a human chromosome (c), two human chromosomes (d), and two (e) and three (f) mouse chromosomes, which are shown in the lower panels.