Cultured skin fibroblast cells were prepared from two patients with mucolipidosis III (ML III), which is a genetic disorder characterized by low activities of multiple lysosomal enzymes in fibroblasts. Genetic complementation analysis of fused fibroblast hybrids revealed that the patients were classified in different complementation groups. Growth curves of fibroblasts of ML III patients in culture were compared with those of fibroblasts of Sanfilippo's syndrome patients as well as of the normal fibroblasts. Normal and Sanfilippo fibroblasts gave essentially the same sigmoid curve of cell growth. However, although both ML III cell lines grew at the normal rate in the initial logarithmic phase, they continued to proliferate actively even after the cultures reached confluency. This is the first report to demonstrate the deficiency of density-dependent regulation of cell growth in the culture of nontransformed cell types. Therefore, the culture of skin fibroblasts of ML III patients may serve as a useful experimental model for investigating the regulation of cell proliferation in vitro.

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