When you have a new technology with so many potential applications (adding 152 extra codons on top of the normal 64), it can be overwhelming to figure out where to start. Getting from "we can add letters to the DNA alphabet" to "we can build new amino acids into IL-2 to facilitate PEG binding to stop IL-2 binding to the alpha unit of the IL-2 receptor while still binding the beta and gamma units, which will preserve anti-cancer effects while sparing vascular damage" seems like a leap that would require a cross-disciplinary team to make
IL-2 is a molecule that is effective at treating a variety of diseases but has nasty side effects that make it a poor drug. The article discusses sometimes fatal side effects of high dose IL-2 in treating cancer; low-dose IL-2 is actually an effective treatment for autoimmune disease, although it is difficult to titrate the dose in such a way that you don't accidentally dose too high and make the autoimmunity worse.
Another startup, Delinia, engineered an agonist selective for the alpha-beta-gamma subtype of the IL-2 receptor to treat autoimmune disease [0] (where Synthorx is hitting only the beta-gamma part of the IL-2 receptor to treat cancer). Delinia was acquired by Celgene last year for $775M, 3 months after its Series A. Synthorx went public this year and has a $450M market cap. All of that from making different versions of a single protein
[0] https://lifescivc.com/2016/09/re-balancing-immunity-via-regu...