By Vincent Dowd
BBC World Service
This weekend sees the 150th anniversary of the birth of Arthur Conan Doyle, the creator of the world's most celebrated fictional detective.
So what's kept him at the top for 122 years?
In 1887, appearing in print for the first time, Sherlock Holmes set out his purpose in life.
The declaration in "A Study in Scarlet" would also come to dictate much of the subsequent career of Holmes' creator, Arthur Conan Doyle - not always to his pleasure.
"There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it."
He went on to define the archetype of the brilliant but troubled detective.
Even today the character of Holmes defines what we expect of great fictional detectives.
We want them to accept that "duty" to do good - but also to be personally flawed.
The 28 year-old author wasn't the first to spot the narrative potential of an incisive but troubled detective.
Conan Doyle himself acknowledged the influence of Edgar Allan Poe's Auguste Dupin and of Lecoq, created by the now largely forgotten Emile Gaboriau.
But almost every fictional detective stands in Holmes' shadow - from Kurt Wallander back to Raymond Chandler's Philip Marlowe.
Chandler once wrote: "Sherlock Holmes is mostly an attitude and a few dozen lines of unforgettable dialogue."
This may or may not have been a compliment.
The Holmes phenomenon really took off in 1891 when he transferred to the just-launched Strand magazine.
In modern media-speak it was the perfect match of platform and content.
Conan Doyle realised reading habits were changing as an expanding middle class enjoyed greater leisure.
But the main demand was for short fiction, to be read in a single sitting or even on a train.
Daniel Stashower, a US biographer, says Conan Doyle spotted a new hunger for continuing fiction in which main characters carry over from one story to the next but which could be read in any order.
"It didn't much matter if you missed an instalment - like a TV mini-series today," he explains.
Another biographer, Andrew Lycett, says Conan Doyle was fortunate in his relationship with The Strand.
The magazine was aimed at an emerging class which 50 years earlier might have been semi-literate and which today would probably watch drama on television.
His timing was perfect in another way too: the cultural influence of the British Empire was at its height - as pervasive as that of America in the 20th Century.
Holmes crossed the globe on the shoulders of imperialism.
Yet none of this would have mattered had readers not already been fascinated by the character of Holmes and by his relationship with Dr Watson.
The double-act was a brilliant stoke, says Andrew Lycett, and has been widely borrowed since.
"Think of Inspector Morse and Lewis - it's a similar pattern," he says.
The Holmes-Watson relationship was also a gift to adapt for stage and later for the screen.
Robert Downey Jr will play the latest film version of Sherlock Holmes
The first Hollywood Holmes appeared as early as 1916 and there had already been German and French adaptations.
Pastiches and parodies of Holmes have long been plentiful.
Daniel Stashower says Conan Doyle created a strong basic set-up which could bear endless reinterpretation.
"So in the 1940s Basil Rathbone battled the Nazis and later on TV Jeremy Brett gave us a man struggling with his inner demons."
Out later this year is the new Guy Ritchie movie, with Robert Downey Jr. as Holmes and Jude Law as Watson.
Hollywood's betting that almost a century after the character was first on screen Holmes remains box-office.
If this version of the relationship clicks with audiences we can expect further outings.
The producers know that audiences almost everywhere have been pre-sold the relationship.
One hundred and twenty two years after his first appearance in print, Holmes offers the modern, content-hungry media what it most craves - a unique and well-established 'brand' to exploit.
Andrew Lycett thinks nostalgia also plays a role.
"People are looking for an era when things were more clear-cut and stable," he says.
"Holmes seems to epitomise a world where crimes can be solved through the use of reason and observation.
"Actually if you read the stories carefully that's often not the case - but it is the perception".