Ficolin-3 (Hakata antigen or H-ficolin) is a soluble pattern recognition 
molecule in the lectin complement pathway. We speculated whether common genetic 
variations in the FCN3 gene contribute to deficiency of Ficolin-3. The FCN3 gene 
was sequenced in 237 healthy Danish Caucasians. The relevance of polymorphisms 
was assessed with antibodies against Ficolin-3 in a novel ELISA system and by 
production of recombinant Ficolin-3 variants. Ficolin-3 serum profiles were 
analyzed by SDS-PAGE and western blotting. Ficolin-3 serum concentration varied 
10-fold (median, 24microg/ml; range, 3-54microg/ml). Out of several 
polymorphisms one FCN3+1637delC causing a reading frame shift and a distortion 
of the C-terminal end of the molecule with an allele frequency of 0.011 was 
particularly interesting. In individuals heterozygous for the FCN3+1637delC 
deletion lowered Ficolin-3 concentration was observed (P=0.025). SDS-PAGE and 
western blotting of serum revealed a weak band corresponding to the truncated 
molecule in addition to the normal Ficolin-3 pattern. Characterization of 
recombinant Ficolin-3 derived from FCN3+1637delC showed that in the homozygous 
situation this allelic variant would lead to Ficolin-3 deficiency. In conclusion 
an FCN3+1637delC deletion variant disrupting the possibility for pattern 
recognition was detected. Characterization of recombinant variant Ficolin-3 
shows that homozygosity for the FCN3+1637delC deletion may lead to Ficolin-3 
deficiency and may thus be the basis for a novel complement deficiency state.