With the emergence of new therapies for spinal muscular atrophy (SMA) — most notably, the antisense oligonucleotide nusinersen — objective longitudinal measures of treatment response are becoming increasingly important. SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene, which result in a lack of functional SMN protein. A team from Japan has now developed a blood test that can differentiate between patients with SMA and healthy control individuals on the basis of SMN expression levels. Using imaging flow cytometry combined with more detailed ‘spot analysis’, the researchers were able to demonstrate markedly reduced SMN levels in CD33++ peripheral blood cells from patients with SMA. This biomarker test can be applied to small volumes (<1.5 ml) of blood and could potentially be used to monitor SMN levels — and, hence, treatment efficacy — in patients who are undergoing therapy for SMA.