There's not much info so this will be a short diary, but this is grave:
Seven people have died in Homs after they inhaled a poisonous gas sprayed by government forces in a rebel-held neighbourhood, activists said.
Activists also told Al Jazeera that scores of others were affected in al-Bayyada neighbourhood. Side effects reported include nausea, relaxed muscles, blurred vision, and breathing difficulties.
More info at al-Jazeera
It should be noted that there is no claim of responsibility, the assumption is that since the attack was on a rebel area it was released by government troops or Assad loyalists. This is the most likely scenario.
That said, sarin has been used in the past in a terror attack on the Tokyo subway in 1995, so this is not necessarily from the Syrian Government stockpile.
This could also be a false-flag attack by rebels or other interested parties like al-Nusra Front, the Syrian branch of al-Qaeda who would like to see the rebels receive military assistance from the US.
Whatever the source, this opens a new and very grave chapter in the liberation of Syria.