Tacrolimus is an immunosuppressive agent that helps prevent rejection in patients that have had organ transplants. It can also be used to treat some skin conditions. It was discovered in 1984 from a fermentation of a Japanese soil sample. This sample contained the bacteria Streptomyces tsukubaensis. Its chemical name is known as macrolide and its shelf name is known as Prograf. It prevent rejection by binding to the FK506 binding protein. This creates a new complex which inhibits calcineurin which inhibits T-lymphocyte signal transduction and IL-2 transcription. This causes the body not to attack the organ and keep it healthy.
The reason this molecule is so important to me is because it keeps me alive on a daily basis. I had a double lung transplant in August 2009 and have been on tacrolimus since day one. Without this molecule I would very easily reject my lungs. So in a round about way tacrolimus has allowed me live my life and follow my dreams. A pretty good reason for it to be my favorite :)
References: http://www.drugbank.ca/drugs/DB00864: http://www.cvs.com/drug/summary/Tacrolimus-Oral-capsule?onId=5658&onType=G&formId=105&DrugInfoPage=Summary&Drug20Name=Tacrolimus-Oral-capsule