This is the first report on the molecular basis of human complement C1s def
iciency. Two abnormalities in the C1s gene were identified in a Japanese fa
mily, including one patient, by using exon-specific PCR, single-strand conf
ormation polymorphism analysis, and nucleotide sequencing, A deletion of 4
bp, TTTG, was identified in exon X when using genomic DNA from the patient,
his father, and his paternal grandmother, They were all heterozygous for t
he mutation. The mutant gene encodes a truncated C1s from the N terminus to
the short consensus repeat domain. By further sequencing the PCR products,
a nonsense mutation from G to T was identified at codon 608 in exon XII in
the patient, his mother, and his sister, They were all heterozygous for th
e nonsense mutation. The mutant gene encodes a truncated form of C1s that l
acks the C-terminal 80 amino acids, These results indicate that the patient
was a compound heterozygote with the 4-bp deletion on the paternal allele
and the nonsense mutation on the maternal allele, The levels of serum C1s s
eem to be correlated to the genotypes of the C1s gene in which no C1s was d
etected in the patient, and one-half of the normal level in the family memb
ers who are heterozygous for either mutation. The present study demonstrate
s that the disease is inherited in an autosomal recessive mode.