although the recent paper mclaughlin m m et al j biol chem 268 6470 1993 reporting the hpde iv b2 sequence has shown by expression cloning that this isoform can produce enzymatically active protein in a yeast expression system mammalian expression clones for hpde iv b1 b2 b3 the hpde iv b1 b2 b3 cdna sequences were subcloned into the mammalian expression vector pcdna1 amp a vector that is suitable for transiently expressing these genes in cos cells and that was constructed by replacing the 950 bp nhei fragment of pcdna1 invitrogen with a 1 2 kb pcr fragment from puc18 sigma containing the amp resistance gene the resulting expression clones are designated pc hpde iv b1 pc hpde iv b2 and pc hpde iv b3 all three clones have been shown to direct the expression of proteins that catalyze the degradation of camp when transiently transfected into cos cells genomic sequences for hpde iv b overlapping genomic clones define 26 kb of genomic sequence encoding at least the 3 u2032 half of the hpde iv b gene fig5 limited dna sequencing of these genomic clones confirms that the sall restriction site in clone u03bbk2 1 is contained in an exon and corresponds to the unique sall site seen in the cdna sequence 1 235 1 240 in sequence id no 1 hybridization data fig5 defines the orientation of the gene and confirms the hybridizing fragment sizes seen in genomic southern blots hybridized at high stringency with the 308 bp pcr fragment bp 1 575.