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Sequence, expression and complementation studies of dSno Ex17B , dSno Ex4B and dSno 174 Since our initial paper on dSno [1], we sequenced the breakpoints associated with our excision alleles (Figure S1). Sequencing revealed that dSno Ex17B is a deletion of 5023bp when compared to the parental dSno sh1402 allele or 11940bp when compared to the D. melanogaster chromosome arm 2L reference sequence due to the presence of the 6917bp 297 retrotransposon in the reference sequence but not dSno sh1402. dSno Ex17B deletes the three dSno promoters we identified previously as well as the neighboring gene CG7231 and part of the next gene CG7228. CG7231 is a gene of unknown function and we found that dSno Ex17B fully complemented four homozygous viable P insertions in this gene-P{EPgy2}EY11884, P{EP}2510, P{SUPor-P}KG04307, P{wHy}DG01605. CG7228 was identified in an cell culture RNAi screen in blood cells as required to permit mycobacterial infection [3]. It has 13 known viable transposon insertions and none have any reported organismal mutant phenotype. Thus, we conclude that the mutant phenotype of dSno Ex17B mutants is attributable to the loss of dSno. This conclusion is supported by evidence that dSno Ex17B mutants can be rescued with UAS.dSno [1]. We examined dSno RNA expression in dSno Ex17B homozygous and transheterozygous mutant embryos (Figure S2) and found a low level of wild type expression Taken together the sequence and expression data suggest that while the dSno open reading frame is present dSno Ex17B acts as a strong hypomorphic allele. Sequencing revealed that dSno Ex4B is a deletion of 20849bp when compared to dSno sh1402 or 27766bp with regard to the D. melanogaster chromosome arm 2L reference sequence. The deletion removes the exon at the amino terminus of all dSno proteins that contains the Smad4-binding amino acids. It also removes the neighboring genes CG7231, CG7228 and CG7227. We conclude that dSno Ex4B is a protein null but that its mutant phenotype likely includes a contribution from the loss of CG7227 (for which no mutants are currently available). This supposition is supported by data showing that homozygous dSno Ex4B mutants cannot be rescued with UAS.dSno [1]. 2 As reported in [2], dSno 174 is a deletion of 9518bp when compared to dSno sh1402. The deletion begins at amino acid 57 removing the remaining 276 amino acids of CG7233 and the splice acceptor creating essentially a dSno protein null. An examination of dSno 174 homozygous …