'You are the best': Mark Ruffalo's birthday wish to 'Avengers: Endgame' co-star Robert Downey, Jr. proves that the 'Science bros' share an equally strong bond off-screen

From Sherlock to Iron Man, Robert Downey, Jr. has never failed to mesmerise viewers with a charismatic performance. Now, as he turned 54 years old on Thursday, April 4, his 'Avengers' co-star Mark Ruffalo extended heartwarming birthday greetings to the actor. He took to Instagram and shared an adorable picture where Downey can be seen planting a kiss on his cheek.

In the post, 'The Hulk' wrote, "Happy Birthday, @robertdowneyjr! Thank you for your kindness, leadership, and generosity for all 50 years. You are the best 💚"

Here's the post:

WATCH: Joe Russo Confirms That The Ultimate Avenger Will Return In 'Avengers: Endgame'

Known as the 'Science Bros' in the Marvel Cinematic Universe, Tony Stark aka Iron Man played by Downey and Bruce Banner aka The Hulk played by Mark Ruffalo share a very strong bond on screen and seems like the bond is equally stronger off-screen as the actors are often seen engaging in hilarious banters on social media.

Meanwhile, the Marvel superheroes are all set to pair up yet again for the fourth and the last venture of the 'Avengers' franchise titled 'Avengers Endgame'. The much-awaited film is all set to hit the big screens on April 26, 2019. 

READ: HILARIOUS: Here's How 'Science-bros' Robert Downey Jr And Mark Ruffalo Came In Rescue To Their 'Avengers' Co-star Chris Evans' Technology Trouble

It is the 22nd instalment in the Marvel Cinematic Universe. The flick brings together all the Marvel superheroes, played by actors including Robert Downey Jr., Chris Hemsworth, Mark Ruffalo, Chris Evans, Scarlett Johansson, Benedict Cumberbatch, Don Cheadle, Tom Holland, Chadwick Boseman, Paul Bettany, Elizabeth Olsen, Anthony Mackie, Sebastian Stan, Letitia Wright, Dave Bautista, Zoe Saldana, Chris Pratt, Jeremy Renner, Evangeline Lilly, Jon Favreau, and Paul Rudd. In fact, Oscar-winning actress Brie Larson who debut in MCU as Captain Marvel recently will also be a part of the film. Meanwhile, actor Josh Brolin will also resume the role of Thanos in the upcoming venture following 'Avengers Infinity War'.

'Endgame' is a direct sequel to 'Infinity War' from the last year. Directed by the Russo Brothers, the film is all set to release in India on April 26, 2019.