Hypothesis

This repeatable quest is given by Clayton Holdren of Bioscience. The task involves retrieving a tissue sample from high level supermutants. He'll give a random location (usually locations used as bases by supermutants) where the player must hunt down the marked super mutant(s). Deliver the sample to him to complete the quest.

Also, you can get tissue samples as a random loot from defeated high level supermutants before even triggering this quest. These are quest items so they don't have any weight and you can't store, sell, or drop them.

Here are the possible locations where the tissue samples can be found:

• Big John's salvage
• Breakheart Banks
• Coast Guard Pier
• Fallon's department store
• Faneuil Hall
• Fraternal Post 115
• Gwinnett Brewery
• Hub 360
• Mass Fusion disposal site
• Medford Memorial Hospital
• Parkview Lounge
• Scrap Palace
• Shaw High School
• Swan's Pond
• Trinity Plaza
• West Roxbury Station
• Wilson Atomatoys Corporate HQ
• Wilson Atomatoys Factory

Source: Fallout Wikia