The primary defect arising from Zellweger syndrome appears to be linked to impaired assembly of peroxisomes. A human complementary DNA has been cloned that complements the disease's symptoms (including defective peroxisome assembly) in fibroblasts from a patient with Zellweger syndrome. The cause of the syndrome in this patient was a point mutation that resulted in the premature termination of peroxisome assembly factor-1. The homozygous patient apparently inherited the mutation from her parents, each of whom was heterozygous for that mutation.