Actinopathies are defined by missense mutations in the ACTA1 gene coding for sarcomeric actin, of which some 70 families have, so far, been identified. Often, but not always, muscle fibers carry large patches of actin filaments. Many such patients also have nemaline myopathy, qualifying actinopathies as a subgroup of nemaline myopathies. This article concerns a then newborn, now 2 1/2-year-old boy, the first and single child of nonconsanguineous parents, who was born floppy, requiring immediate postnatal assisted ventilation. A quadriceps muscle biopsy revealed large patches of thin myofilaments reacting at light and electron microscopic levels with antibodies against actin but only a few sarcoplasmic rods and no intranuclear rods. DNA analysis of the patient's and both parents' blood did not reveal any missense mutation in the ACTA1 gene. Thus, this congenital myopathy can be caused by a new type of ACTA1 gene mutation, a new non-ACTA1 gene mutation, or no mutation at all, designating it as an actin-related myopathy, perhaps a new type of congenital myopathy and a new member of protein aggregate myopathies marked by aggregation of proteins within muscle fibers, among them desminopathies, alpha-beta crystallinopathies, other desmin-related myopathies (also termed myofibrillar myopathies), actinopathies and, now, actin-related myopathies.