A hardgainer is a person who practices bodybuilding but finds it challenging to develop musculature regardless of the amount of effort put in.
[2][failed verification] Difficulty building muscle is often associated with the ectomorph body somatotype, however other common reasons also include a lack of proper nutrition, suitable physical activity level or not allowing enough recovery time for the stressed muscles to regain their previous state and then grow bigger (overtraining).
[3][failed verification] For the true hardgainer, the issue lies deeper beneath any of the required elements of muscle gain listed above.
Those are typically either diseases that affect muscles and/or protein synthesis, or there might be a genetic disorder that hinders protein synthesis and/or limits the maximum amount of muscles the body can hold to a relatively small amount for that person.
There are no known health problems related to the myostatin mutations, and affected individuals are intellectually normal.