Aliens have arrived to Earth from the depths of space, and thankfully they are peaceful and only want to learn about us and interact with us.
Despite some initial trepidation, first contact goes well and the Aliens quickly learn our languages and offer an exchange of cultural and scientific knowledge. These Aliens are quite friendly and they seem quite taken with us Human Beings. Inter-species friendships soon start forming, and despite some opposition from those who do not quite trust our new friends, a peaceful political alliance is formed between the United Nations and the Aliens.
Some of us, who are particularly curious, end up in very amorous relationships with these Aliens. Like us, these Aliens have quite a high sex drive and many of them find us Humans quite attractive. Many people find the idea of sex with Aliens to be quite odd or even disgusting, but for some people: that is their kink and they love it.
It transpires that not only is mating possible between a Human and an Alien, there is also potential for procreation. Complications are abound in the successful delivery of a hybrid child, but with the combined medical and technological knowledge of both us and the Aliens we are able to make it happen.
A whole sequence of events unfolds from not only the presence of these friendly Aliens in our society but also the creation of this new hybrid race.
But what I am interested in is, speaking in terms of both appearance and genetics, how different can these Aliens really be from us, if mating and child birth is possible between the species.
I am assuming the Aliens would have to be pretty close to our DNA and fairly close to us in appearance - but I want to make them as Alien as possible whilst still allowing for inter-species breeding.
So the question is:
How different from us can we make the Aliens whilst at the same time ensuring that procreation is possible between the species?
I have left some things intentionally vague to give people some room to get creative. For instance, I am not dictating whether the children can be born in either a Human or Alien womb or both or neither - I am only stipulating that procreation is possible (although successful births can be difficult). I am also not stipulating the types of genders for the Aliens - they can have male and females sexes like us, but if you can find a way for singled-sexed or 3-sexed (or whatever) to work, go for it. The more different they can be the better.
The big green tick will go to the answer that can show the biggest difference(s) between these Aliens and us whilst at the same time demonstrating that inter-species intercourse and breeding is possible.
It has already been established by many here that the Aliens would have to be extremely similar to Humans in order for them to be able to procreate. This isn't an issue for my story and stating this doesn't really answer the question. Even if the differences are minor, please give examples of what they could be - if it turns out that the biggest difference can only be something like "the Aliens have purple eyes" then this is an acceptable answer (and preferable to being told once more about Chimpanzees).