Objective: To determine the phenotype variability associated with the specific C-terminal M-line titin mutation known to cause autosomal dominant distal myo pathy, tibial muscular dystrophy (TMD; MIM 600334), and limb girdle muscular dys trophy 2J (LGMD2J). Methods: Three hundred eighty-six individuals were genotype d for the Finnish founder mutation in titin (FINmaj) causing TMD/LGMD2J. Results : Two hundred seven patients were heterozygous for the mutation. Among these pat ients, 189 (91%) had a more common phenotype compatible with the classic descri ption of TMD. However, 18(9%) had unusual phenotypes such as proximal leg or po sterior lower leg muscle weakness and atrophy even at onset.Four patients were c onfirmed homozygotes representing the LGMD2J phenotype. These homozygotes were h alf of the eight LGMD patients previously described in the original large consan guineous kindred. Conclusions: Large variability of phenotypic expression caused by just one mutation, the Finnish FINmaj, suggests that no certain phenotype of myopathy/dystrophy can be excluded from being caused by mutated titin. Yet unkn own homozygous or compound heterozygous titin mutations without phenotype in the heterozygote carriers may be responsible for undetermined recessive MD and LGMD . Objective: To determine the phenotype variability associated with the specific C-terminal M-line titin mutation known to cause autosomal dominant distal myo pathy, tibial muscular dystrophy (TMD; MIM 600334), and limb girdle muscular dystrophy 2 J (LGMD2J). Methods : Three hundred eighty-six individuals were genotype d for the Finnish founder mutation in titin (FINmaj) causing TMD / LGMD2J. Results: Two hundred seven patients were heterozygous for the mutation. Among these patients, 189 (91%) had a more Common, phenotype compatible with the classic descrition of TMD. However, 18 (9%) had unusual phenotypes such as proximal leg or po sterior lower leg muscle weakness and atrophy even at onset. Four patients were c onfirmed homozygotes representing the LGMD2J phenotype. These homozygotes were h alf of the eight LGMD patients previously described in the original large consan guineous kindred. Conclusions: Large variability of phenotypic expression caused by just one mutation, the Finnish FI Suggests that no certain phenotype of myopathy / dystrophy can be excluded from being being by mutated titin. Yet unkn of homozygous or compound heterozygous titin mutations without phenotype in the heterozygote carriers may be responsible for undetermined recessive MD and LGMD.