Functional proliferation of peripheral blood mononuclear cells (PBMCs) was assessed using dye labeling, CD3/CD28 activation, and flow cytometric analysis.<h4>Results</h4>Three female siblings with COPB1 deficiency presented with early-onset cataracts, global developmental delay, hypotonia, and progressive spasticity leading to quadriplegia. This evidence concerns the gene CD28 and cataract.