Here is what Sabisky is alleged to have said:
Yesterday, reporting by The National revealed that the 27-year-old once claimed it was a woman’s Christian “duty” to have sex with her husband.
Sabisky made the comments on the popular forum site Reddit, where he frequently gave advice to other users on rape fantasies, incest, and faith.
Writing under the name “thedovelamenting” - an apparent reference to the Book of Isaiah - Sabisky told a man to “rewire” his wife’s brain “to the point where she no longer, consciously or subconsciously sees a conflict between a good Christian woman and serving you up 39 flavours of slut on command.”
He wrote: “Theologically speaking she is your wife and should submit to you as unto the Lord, so if you want doggy then it is your place to command her to get on her hands and knees and her place to obey.”
And in a now-deleted Twitter post from last year, Sabisky wrote: "I am always straight up in saying that women's sport is more comparable to the Paralympics than it is to men's."
According to The National, when a woman on Reddit posted about finding “disturbing porn” on her boyfriend’s computer, including erotic fiction about incest, paedophilia and rape, Sabisky told her he didn't “see all that much to worry about here”.
“Fantasies are strange things,” he wrote, “Unpredictable, of uncertain origin, hard to figure how they’ll relate to real life. I have a thing for incest erotica myself despite never having had those fantasies relating to my actual family.
“So long as he’s not actually breaking any laws and is restricting his viewing habits to girls 18 and up I wouldn’t be too squicked,” he advised.
In a 2014 comment under a post on an education blog, someone using the screen name Andrew Sabisky suggested that African Americans have lower average IQ scores than white Americans.
In a comment on a blog from 2014, a user with the same name as Mr Sabisky claimed: "There are excellent reasons to think the very real racial differences in intelligence are significantly - even mostly - genetic in origin, though the degree is of course a very serious subject of scholarly debate."
In a comment on Mr Cummings’ own website in 2014, a user called “Andrew Sabisky” argued for compulsory contraception to be enforced.
“One way to get around the problems of unplanned pregnancies creating a permanent underclass would be to legally enforce universal uptake of long-term contraception at the onset of puberty," they wrote.
"Vaccination laws give it a precedent, I would argue."
In a 2016 interview with Schools Week, which described Mr Sabisky as a “polymath”, the then 24-year-old discussed eugenics.
“Eugenics are about selecting ‘for’ good things,” he is quoted as saying, “Intelligence is largely inherited and it correlates with better outcomes: physical health, income, lower mental illness. There is no downside to having IQ except short-sightedness.”
What did Sabisky say about the alleged comments?
Sabisky, who had been in the No 10 role for less than two months, did not apologise for the alleged comments, calling the reporting “a giant character assassination”.
In a tweet on Monday evening, he wrote: “I wanted to help the government not be a distraction… accordingly I’ve decided to resign.”
“I know this will disappoint a lot of people but I signed up to do real work, not be in the middle of a giant character assassination.”
“If I can’t do the work properly there’s no point, and I have a lot of other things to do with my life.”
Downing Street under fire
When asked on Monday, Boris Johnson’s spokesperson refused to condemn Sabisky’s alleged remarks.
Asked repeatedly whether the prime minister agreed with them, the spokesperson told journalists: "The prime minister's views on a range of subjects are well publicised and documented."
In 2008 Johnson apologised for an article published in the Spectator magazine which claimed black people have lower IQs while he was editor.
In one, Spectator columnist Taki claimed that “Orientals...have larger brains and higher IQ scores. Blacks are at the other pole.”
Johnson, then a London mayoral candidate, apologised for publishing the article and said: “I am sorry for what was previously written as it does not reflect what is in my heart.”
How did Labour react to his resignation?
Labour, who had called for Sabisky to be fired, welcomed his resignation.
Party chairman Ian Lavery said: "It's right that Andrew Sabisky is no longer working in government.
"After No 10 publicly stood by him today, Boris Johnson has serious questions to answer about how this appointment was made and whether he agrees with his vile views."