MASS phenotype is a familial connective tissue disorder similar to Marfan syndrome that affects different people in different ways. MASS is an acronym for features of the disorder that may be present:

• Mitral valve prolapse - a heart condition in which the two valve flaps of the mitral valve in the heart do not close smoothly or evenly, and bulge (prolapse) upward into the left atrium.
• Aortic root dilation - borderline, non-progressive enlargement of the section of the aorta where the aorta and heart meet. Unlike in Marfan syndrome, aortic complications such as dissection and aneurysm usually do not occur. However some authors believe there may be an increased risk for aortic complications.
• Skin striae (stretch marks) - skin may have stretch marks even without changes in weight.
• Skeletal features - possible features include curvature of the spine (scoliosis), chest wall deformities, and joint hypermobility.

People with MASS phenotype also have nearsightedness (myopia), but they do not experience eye lens dislocation (ectopia lentis) as in Marfan syndrome.

MASS phenotype has rarely been diagnosed in people with a mutation in the FBN1 gene (which usually causes Marfan syndrome), but in most cases the cause of MASS phenotype is not yet known. People diagnosed with MASS phenotype who do have a FBN1 mutation may later be diagnosed with Marfan syndrome as additional features develop, which would convey a high risk of aortic complications. It may be hard to distinguish MASS phenotype from early features of Marfan syndrome in young individuals.

Management focuses on the specific symptoms and severity in each person. It is important to monitor signs and symptoms over time, such as having annual heart and eye exams. This is especially important for people under 20 years of age who potentially have Marfan syndrome, since additional features may develop with age.