Mosaicism (a mixture of normal cells and cells with an abnormal chromosome) is rarely seen in multiple family members. We studied a family spanning three generations that had an abnormal chromosome 8 with a small piece of chromosome 21 attached to its end (der(8)). Multiple family members also had cells with normal chromosomes resulting in different levels of mosaicism. Genetic testing showed that the instability of the genetic code sequence where the piece of chromosome 21 was joined to chromosome 8, likely caused loss of the extra piece of chromosome 21 thus changing the der(8) chromosome back to a normal chromosome 8. (By Dr. Weimin Bi, https://jmg.bmj.com/content/early/2022/09/23/jmg-2022-108586 )