SFTPB and hyperinsulinemic hypoglycemia, familial, 4: 3). Based on our results, the biochemical analysis of BAL fluid for mature SP-B, previously thought to be diagnostic for SP-B deficiency, is not 100% specific, as there are additional cause(s) leading to a lack of SP-B. Possible mechanisms include mutations or secondary changes in regulatory elements or other defects in the synthesis and secretion of surfactant, as recently shown for the ABCA3 transporter [27].