Here, we report the biochemical and genetic basis of the Vel blood group 
antigen, which has been a vexing mystery for decades, especially as anti-Vel 
regularly causes severe haemolytic transfusion reactions. The protein carrying 
the Vel blood group antigen was biochemically purified from red blood cell 
membranes. Mass spectrometry-based de novo peptide sequencing identified this 
protein to be small integral membrane protein 1 (SMIM1), a previously 
uncharacterized single-pass membrane protein. Expression of SMIM1 cDNA in Vel- 
cultured cells generated anti-Vel cell surface reactivity, confirming that SMIM1 
encoded the Vel blood group antigen. A cohort of 70 Vel- individuals was found 
to be uniformly homozygous for a 17 nucleotide deletion in the coding sequence 
of SMIM1. The genetic homogeneity of the Vel- blood type, likely having a common 
origin, facilitated the development of two highly specific DNA-based tests for 
rapid Vel genotyping, which can be easily integrated into blood group genotyping 
platforms. These results answer a 60-year-old riddle and provide tools of 
immediate assistance to all clinicians involved in the care of Vel- patients.