Monoclonal anti-lih's and monoclonal anti-Gm's are now available, perm
itting a more precise serological determination of the human allotypes
. Several Gm allotypes can be determined at the gene level, without re
sort to serological reagents. PCR, subclass-specific amplification and
allele-specific probes are used. Many RFLPs of the human IgH C- and V
-region genes have now been defined. More than 10(6) variant haplotype
s of the so-called constant part of the Ig chain are possible because
more than 20 polymorphic sites are known. A schematic physical map of
the human IgHC gene cluster is presented.