1. y; bw cn sp flies are cultivated in cages with apple juice agar plates covered with yeast powder. 2. For a 16-24 hours egg laying, new plates are added after a 2 hours pre-egglaying at 12 a.m. and flies are allowed to lay eggs for 8 hours. 3. Embryos are collected at 7.45 a.m. using a filter mesh and a brush. They are rinsed with EWB and cross-linked at 8 a.m. in 1.8% formaldehyde at room temperature for 15 min. Solutions and MaterialsLysis buffer: (see above)5mg/ml RNaseA (DNase free)PAS suspension: 100 mg of CL-4B (Amersham, 17-0780-01) PAS shoµld be resuspended in 1 ml of lysis buffer + protease inhibitors + 0.1 mg/ml BSA for 50 %v suspension. Wash in lysis buffer 2-3 times, overall equilibration time 1h. Store up to one week at 4 C.TE: 10 mM Tris-HCl pH 8.0, 1 mM EDTAElution buffer1: 10 mM EDTA, 1% SDS, 50 mM Tris-Cl pH 8.Elution buffer2: TE+0.67% SDS.LiCL 4M? Protocol 1. To an amount of chromatin corresponding to 150 mg of biological material, suspended in a final volume of chromatin extract of 1 ml (in lysis buffer + protease inhibitors), add 100 µl of PAS suspension for preincubation. Incubate several hours or overnight at 4°C, then remove PAS. Crosslinked chromatin at this stage can be stored several days at 4°C or frozen at ?70°C.2. [Optional : Check for amount of DNA. From the 1 ml solution above, take a 100 µl aliquot, Add proteinase K up to 100 ug/ml and SDS to 1%, incubate 6h at 60°C, then 20 min at 70°C, add RNAse to 50 ug/ml and incubate additional 2h at 37°C. Phenol-chlorophorm extract, ethanol precipitate. Run on an agarose gel to check amount and size of DNA.]3. Separate chromatin sample into 4x250 µl aliquots (one aliquot is enough for one IP but the amount may be decreased further). Immunoprecipitate chromatin by adding the antibody (Ab) of interest (amount of Ab shoµld be determined empirically, usually the same concentration than for a successfµl IF experiement). Keep a control sample without Ab IP, named "Mock".4. Incubate 4 h at 4°C on a head to head rotating wheel, then add 50 µl of PAS suspension and incubate 4 h or overnight. Spin down PAS and proceed to washes.5. Wash PAS 4x with lysis buffer, followed by 2x with TE (without protease inhibitors). Each wash is for 5 min at 4°C, using 1 ml of solution.6. Elution of precipitated material. Spin down PAS. Add 100 µl of elution buffer, mix and incubate 10 min at 65°C. Spin down PAS and transfer supernatant to new tube. Add 150 µl of elution buffer 2 to PAS, mix, centrifuge at fµll speed and transfer eluate to a tube together with the eluate from the first centrifugation. The combined material is the "chromatin precipitate" (approx. 250 µl).7. Incubate precipitate 6 h (or overnight) at 65 C to reverse cross-links. Add 250 µl of Proteinase K solution, incubate at 50°C 2-3 h.8. Add 55 µl of 4M LiCl and 500 µl of phenol-chlorophorm. Mix and microfuge at fµll speed at RT. Transfer aqueous phase to a new tube and precipitate with 1 ml of 100% ethanol. Wash with 750 µl of 70% ethanol. Spin down and dry precipitate.9. Dissolve in 25 µl of water. This is the chromatin immunoprecipitate or ?Native ChIP? sample. DNA fragments recovered following chromatin IP prepared for Illumina sequencing using the Epicentre Nextera DNA Sample Prep Kit (Cat. # GA0911). After a final gel purification step the DNA is loaded into a flow cell for multiplexed sequencing.