Cindy Crawford is ''so proud'' of her son Presley.

The 51-year-old supermodel paid tribute to her son as she marked his 18th birthday with a sweet throwback picture on Instagram.

She wrote on the photo-sharing site: ''Happy 18th birthday @PresleyGerber! Seems like yesterday that you were getting puppy love from our little Sugar!

''Loving you always and wishing you a great year ahead. We are so proud of you! (sic)''

Meanwhile, Presley's sister Kaia Gerber previously admitted she loves to ''teach'' her brother about the fashion industry.

She said: ''I love it because I just see him doing so well. He doesn't really know much about the industry, so I get to teach him. He'll be like 'oh, I met this person,' and I'm like 'Presley, do you know who that is? It's this designer or that designer,' so I'll be on set and introduce him to people.''

And Kaia loves working alongside her brother and is thrilled when the pair get the chance to work together because they can support one another when shooting new campaigns.

She explained: ''I love working with him and every time we book a job together I get excited. At first when we were starting out, we used to shoot a lot together and we would just be there for each other, which is important, especially just starting in such a scary industry - to have someone there that you're comfortable with.''

And Cindy previously confessed she doesn't understand social media but thinks the way her children use it is ''cool''.

Speaking in 2015, she said: ''It's so funny, I just don't understand that whole world yet. But, I guess yes and no. Just the way the world works - for me when he started taking pictures of her, the greatest thing about that was that they were doing something together, and I thought it was cool because he's her brother.''