"Conventional" (i.e. phophoramidite) DNA synthesis has a coupling efficiency of ~99% - so the yield after 100-200 bases is about zero. It's been possible for a

"Conventional" (i.e. phophoramidite) DNA synthesis has a coupling efficiency of ~99% - so the yield after 100-200 bases is about zero. It's been possible for a long time, though, to splice together these short stretches to get longer stretches. But that process has been very low yield and required a lot of skill. As a demonstration, 20 years ago Cello et al demonstrated synthesizing an active polio virus (https://pubmed.ncbi.nlm.nih.gov/ https://pubmed.ncbi.nlm.nih.gov/) Although it might be easier to make long stretches of DNA with these machines, it would still take a lot of skill to translate these sequences into a weapon. Anyone with those skills could have already been doing this.