The present study suggests the following points: i) the patient had the 121del2 mutation in intron 4 of the SP-B gene; ii) the patient had partial SP-B protein deficiency and iii) the 121del2 mutation in intron 4 of the SP-B gene may cause partial SP-B deficiency, eventually leading to almost irreversible hypoxic respiration. Here, SFTPB is linked to hyperinsulinemic hypoglycemia, familial, 4.