Abstract: Anabaena sp. PCC 7120 was mutagenized with trans poson Tn5-1087b and screened for mutants unable to grow on dinitrogen and alter ed in heterocyst pattern. Mutant #1801 forms no visible heterocyst within 24h after nitrogen stepdown, and a small number of mature heterocysts within 48h, which a mount to 2.8% of total cell number. The mutant is significantly different from the wild type in rate and frequency of heterocyst differentiation. The flanking g enomic DNA and the transposon were retrieved from the mutant by restriction of i ts total DNA with ClaI, recircularization and electroporation intoE.coli.A s shown with sequencing, the transposon was inserted between Nt. 148 and 149 of alr0099, a gene of unknown function. To prove that the phenotype of the muta nt was indeed due to the insertion withinalr0099 but not a secondary mutati on, we inserted C. K2 cassette, a neomycin resistance marker, into the cyanobact erial genome at the EcoRV site ofalr0099 via double h omologous recombinatio ns. The reconstructed mutant DR214 showed the same phenotype as #1801. These res ults indicate thatalr0099 or its downstream genes sta nd for a novel region involved in heterocyst development and pattern formation in the genome ofAnab aena sp. PCC 7120.