Some viruses have a mysterious 'Z' genome

In 1977, a group of scientists in Russia first discovered that a cyanophage, or a virus that invades a group of bacteria known as cyanobacteria, had substituted all of its As for the chemical 2-aminoadenine (Z). In other words, a genetic alphabet that typically consists of ATCG in most organisms on our planet was ZTCG in these viruses.

https://www.livescience.com/phages-virus-z-genome-more-widespread-than-thought.html