Class 3 infants - Clinically determined but genetically undetermined SMA: newborns/infants with SMA-related symptoms, without a diagnostic genotype for SMA (e.g., no deletion/conversion detected by NBS or heterozygous for SMN1 deletion/conversion), and a variable number of SMN2 copies (Figures 2, 3). The gene discussed is SMN1; the disease is proximal spinal muscular atrophy.