Background:  Variant-specific disease mechanisms, where different variant(s) in the same gene cause different disorders, are increasingly being identified in human monogenic disease. We recently identified a homozygous FICD variant, p.(Arg371Ser), in 5 individuals with neonatal diabetes (NDM), cataracts and developmental delay (DD). We hypothesised that this phenotype was variant-specific as a different recessive FICD variant, p.(Arg374His), causes spastic paraplegia-92. The clinical features of spastic paraplegia-92 did not overlap with those in our 5 cases. Biochemical studies predicted both variants to similarly affect FICD’s function. It is therefore unclear how these variants cause distinct phenotypes. Here, we further explore whether the FICD p.(Arg371Ser) variant causes NDM in a variant-specific manner. Material and methods We screened for rare (GnomAD MAF <0.01%) biallelic FICD variants in 384 probands with diabetes diagnosed <3years of unknown genetic cause using genome sequencing. Results We identified four additional individuals homozygous for the FICD p.(Arg371Ser) variant, taking the total to 9. 8/9 (including 5/5 original cases) had the same nucleotide change (NM_007076.2:c.1113G>C). Importantly, the ninth case harboured a different nucleotide change (NM_007076.2:c.1113G>T). Clinically, age at diabetes onset ranged between 12 weeks and 2 years (median=29.5 weeks). Early-onset cataracts (4/9) and DD (5/9) were common features. No spastic paraplegia-92 features were reported.