Why was Mycoplasma mycoides JCVI-syn1.0 considered the first functioning cell with a synthetic genome?

Mycoplasma mycoides JCVI-syn1.0 was considered the first functioning cell with a synthetic genome because it was the first instance in which a large, chemically synthesized genome—consisting of 1.08 million base pairs—was successfully inserted into a bacterial cytoplasm to create a living, self-replicating organism.1

While earlier research involved assembling smaller synthetic genomes, such as the modified M. genitalium JCVI-1.0, those were not integrated into a cell to produce a functioning life-form. The creation of JCVI-syn1.0 demonstrated that a genome designed and built in the laboratory could effectively "boot up" and control the biological processes of a cell. To distinguish this artificial creation from its natural counterpart, researchers incorporated specific genetic "watermarks" into the DNA sequence. This milestone represented a significant advancement in synthetic biology, moving beyond the modification of existing genes toward the synthesis of entire genomes.1