Accessing Genoscope by trainw

From Paris

First you have to get to the Gare de Lyon--this is relatively easy on the metro or RER (Regional Rapid Transit). There are trains leaving for EVRY-COURCOURONNES (the station for Genoscope) about every twenty minutes (for schedule, see SNCF).
Head for the "Departs Banlieue" (suburban trains). Look at the departure board for trains heading for CORBEIL or CORBEIL-ESSONNES, via EVRY-COURCOURONNES or simply COURCOURONNES (All or any of these may be abbreviated on the departure board). This board will tell you the track (voie, indicated by a number) and it indicates (not too clearly) the stations where the train stops. Before you get on the train, you can confirm that it stops at Evry-Courcouronnes, as there should be a departure board next to the train listing all the stops. The trip usually lasts just over half an hour, and the station just before Evry Courcouronnes is ORANGIS-BOIS DE L'EPINE.
When you get off the train; take the SORTIE PRINCIPALE, Cours Blaise Pascal, which is located at the far end of the platform (front of the train). This is not what seems to be the main exit. When you exit the station, you should see the Credit Lyonnais on the opposite side of the street. Turn left as you leave the station (don't cross the street) on Cours Blaise Pascal, and go to the end of the street; you will reach a main road, Bd. des Coquibus. Turn left on this street. It is about a ten-minute walk along the Bd. des Coquibus to CNS/Genoscope. You will have to cross the street at rue Gaston Cremieux (signal) to the 4-story building with a parking lot in front.

Caveats

If, instead of a track, the departure board at Gare de Lyon indicates "GL", it means the train is leaving from the "Grands Lignes" platform on the surface (the suburban trains usually leave from the basement). In this case, follow the signs to "Grands Lignes" and look at the departure board there to find out the platform letter.
There is a bifurcation in the track to Corbeil at Juvisy, and the trains take two different routes, only one of which stops at Evry-Courcouronnes. All these trains end up at Corbeil, however, so all you have to do is wait for the next train going in the opposite direction (making sure it stops at Evry-Courcouronnes). If you want to avoid this mistake, ask someone in the train if it stops at Evry-Courcouronnes (they might not know), and if it doesn't, get off at Juvisy and wait for one that does. Theoretically there are departure boards next to each train, listing all the stops.
Be sure to take the Sortie principale/Cours Blaise Pascal. If you take the wrong one it might be better to go down to the platform again and take the suggested exit.
If you want to take a taxi, take the exit from the middle of the platform, with the taxi icon. Taxis are not always available.