Altered DHS1P to S1P ratio in Sk1 mutants. (A) The targeting vector used to generate Sk1 mutants by ends-out homologous recombination. (B) sk1 mutants do not show Sk1 protein, indicating that they are null mutants. (C) Quantification of Sk1 in overexpressors and sk1 mutant alleles. (D) Rh1 and TRP are low in 1-d-old sk1¹ compared with control cinnabar.brown (cn.bw) flies. (E) Quantification of Rh1 and TRP shows that Rh1 is decreased to 30% and TRP to 40% in sk1 mutants compared with control cn.bw flies. (F) Rh1 and TRP in sk2 compared with control flies. (G) Quantification of Rh1 and TRP bands in sk2. (C, E, and G) n = 3; bars denote standard deviation. (H) Mass spectrometric measurements show a 20% reduction in S1P level in sk1. n = 3; bars denote standard error. OE, overexpressor.