Mark Sienkiewicz was one of the original developers of the JWST ETC. He designed and performed the initial development of the webserver component of the ETC, and contributed substantially to the design of the user interface. His contributions were critical to the success of the project.

Mark was diagnosed with metastatic cancer a few months before he started work on this project. He knew that this would be his last software project, and that he was unlikely to see the launch of JWST or of the ETC itself.