Titin, is definitely the largest protein in the body, with a molecular weight of 3 million Dalton and composed of 27,000 amino acids. Paradoxically, this huge structure was elusive until the last decade but, since it was described in muscle tissue, its importance has rapidly emerged. Titin constitutes about 10% of muscle mass, to represent the third most abundant protein in the muscle following actin and myosin. It is estimated that titin acts like a "ruler" that controls the relative positioning of the latter 2 muscle proteins, and regulates the flexibility and "springiness" of the contracting muscle. Titin has also been implicated in the condensation of chromosomes during mitosis, while induced mutations in titin caused enhanced fragility of chromosomes. A recent demonstration of a high titer of autoantibodies to titin in sera of patients diagnosed with myasthenia gravis, is interpreted as a prognostic parameter to indicate a severe course of the disease.